For high school (Collège St-Alexandre), probably Alain Vigneault, head coach of the Vancouver Canucks.
For my undergraduate alma mater (University of Ottawa), I don’t know. There’s a list with quite a few well-known people.
ALEX TREBEK, Obviously! Oh-em-gee. 
I’m going to U of O next year, but I just finished seven great years across the river at Carleton University. As far as someone relevant to Americans (and therefore a true celebrity ;)), Dan Aykroyd, and Peter Jennings.
High school: Ron Sega, who may not be famous or the most famous but he’s an ASTRONAUT so he wins.
College: Either Drew Carey or Josh Cribbs or Joe Walsh. Probably Joe Walsh - who got his honorary doctorate at the same time I got my bachelor’s 
High school, Michelle Pfeiffer.
Chris Carpenter of the Cardinals grew up in my town but when to a private high school instead. I don’t think anyone famous actually went to my high school…
College:
author John Irving
I was going to say the same thing. From my other alma mater (University of South Florida), we have Hulk Hogan (Terry Bollea), Lauren Hutton, and Aasif Mandavi.
As for high school, this guy was in the class ahead of mine; he’s a nice guy with an interesting sense of humor.
Undergrad: both Joe Biden and I were history majors at the University of Delaware (not at the same time – he graduated the year I was born); I was the outstanding senior history student in my year, but Joe finished close to the bottom of his class overall. 
George Thorogood also went there.
Grad school: University of Virginia, so there’s a lot of famous alumni as you can see from this handy list
but my favorites are two who didn’t finish their course: Edgar Allan Poe on the one hand, and this one guy whose name escapes me, who flunked out in the 19th century and was told he would never amount to anything. He went on to become quite wealthy and successful, and bought land on a hill that overlooks the school. He built a huge house there; you can’t miss it if you’re stood down near Alderman Library. He did that on purpose. We used to call it the ‘Spite House,’ and sometimes people mistake it for Monticello. (When Jefferson lived at the actual Monticello he did have a clear view down the campus towards the Rotunda and would watch campus activity through a telescope, but trees and subsequent buildings, notably Cabell Hall, put an end to the sight-lines)
I prefer the UD, myself, and rarely talk about my time at UVa (or even tell people I went there, now that I think about it.)
HS-
Tom Cruise
Under Grad-
Andrea Anders
Avery Brooks
Kristin Davis
Calista Flockhart
James Gandolfini
Robert Menendez (Sen)
Robert Torricelli (ex Sen)
Jim Florio (ex Gov)
Mike Taibbi
David DeJesus (KC Royals)
Brian Leonard (St. Louis Rams)
Shaun O’Hara (NY Giants)
L.J. Smith (Philadelphia Eagles)
Gary Brackett (Indianapolis Colts)
Deron L. Cherry (Jacksonville Jaguars)
Grad-
Yogi Berra
Allen Ginsberg
Sharpe James
Dania Ramirez
Bruce Willis
(FTR- HS-
Tom Cruise
Under Grad-
Andrea Anders
Avery Brooks
Kristin Davis
Calista Flockhart
James Gandolfini
Robert Menendez (Sen)
Robert Torricelli (ex Sen)
Jim Florio (ex Gov)
Mike Taibbi
David DeJesus (KC Royals)
Brian Leonard (St. Louis Rams)
Shaun O’Hara (NY Giants)
L.J. Smith (Philadelphia Eagles)
Gary Brackett (Indianapolis Colts)
Deron L. Cherry (Jacksonville Jaguars)
Grad-
Yogi Berra
Allen Ginsberg
Sharpe James
Dania Ramirez
Bruce Willis
(…Lordy, you think I actually know any of these people? If a single one of them ever met me & actually remembers me and my name and my ugly mug, I’ll owe them a steak dinner.)
Double posted w/i edit window, outside of 5 min limit. If a Mod could delete the double portion, I’d appreciate it.
UC Santa Barbara:
Michael Douglas, Benjamin Bratt, Jack Johnson
Gwyneth Paltrow also attended, but did not graduate
HS: No one that I’m aware of
ETA: Wait, no, Marion Ross (Point Loma HS in San Diego)
High school: Well, this is kind of sad–I went to a high school in the middle of nowhere, and with about 70 kids per graduating class, there aren’t a lot to choose from. Wikipedia claims the most famous person to graduate from our school was D. J. Flick. You know, the guy from the Saskatchewan Roughriders. He caught a touchdown pass in the Grey Cup once. Surely you remember him.
College (undergrad): Penn State. You can pick your favorites from here. Personally I’d pick Fred Waring as our most famous alum; it’s not everybody who both led the most noted big band of his era and invented the electric blender.
College (master’s): University of Toronto. You know there are a lot when the Wikipedia page has a separate section for “Prime Ministers.” I guess here you’d have to decide what “famous” means. John Kenneth Galbraith and William Mackenzie King vs. David Cronenberg and Lorne Michaels, for example.
College (doctorate): Jesus College, Oxford. Harold Wilson, Lawrence of Arabia, and, er, Magnus Magnusson.
From Willingboro (NJ) High School: Carl Lewis, track star and Olympic gold medal winner. We were there at the same time, though I didn’t know him. His mom was my gym teacher once.
From Virginia Tech: Michael Vick. He was there many years after I graduated.
Tim Krumrie, former pro-bowl starting nose tackle for the Bengals played ball and graduated from my high school.
Graduate School, in terms of sheer number of fans, newspaper articles, google hits, and biographical books: Batman (Princeton). He beats Kennedy, Wilson, and Michelle Obama on google hits (in fact, he beats them combined).
He wasn’t an alumni, but he did teach English at my high school for a few years in the 30s, long before I was there, too!
Other alumni:
Julian Bond
Stephen Sondheim
Blythe Danner
George Segal
Keir Dullea
You went to Sandy Hook?
Keith’s father and my father were first cousins. I can remember playing with them some when I was just a Little Hook.
Cleveland Heights High School: Dr. Martin A. Samuels. Marty was a classmate of mine (class of '63). He was the Class President, as well as the Student Council President. In fact every single thing he did, he did superlatively. He was the kind of kid every parent wishes for.
THE Ohio State University: I guess the most famous were Jesse Owens and James Thurber. (Honorable Mention: Jeffrey Dahmer, who was able to become famous without even graduating.)
High School - John Hughes I would guess.
College - Neil Armstrong. Purdue.
Sally Struthers was in my sister’s class in high school.
Tom Grant, the jazz musician, also graduated from my high school, the next year.
That’s all I got.
Roddy
U. Saskatchewan - John Diefenbaker, 13th Prime Minister of Canada; Emmett Hall, Justice of the Supreme Court of Canada and major contributor to the development of national Medicare; Willard Zebedee Estey, Justice of the Supreme Court of Canada.
Queen’s - Supreme Court Justice Cromwell; Governor General Johnston; Premier McKenna of New Brunswick
Université Laval - Brian Mulroney, 18th Prime Minister of Canada, and Jean Chrétien, 20th Prime Minister of Canada; H-E Taschereau, Chief Justice of Canada; Louis-Alexandre Taschereau, Premier of Quebec