They might not be taking communion because they have an unconfessed mortal sin. And I don’t believe there is anything in Catholic church teachings that even a Catholic attending mass without an unconfessed mortal sin must take communion. Thus there are reasons other than not being Catholic why a student might not take communion.
They would not be so required at my Catholic high school.
I went to a Catholic school in a moderately wealthy suburban area, and there were 2 non-Catholics in my class of 18 and about the same number in each of the other classes (K-8). In contrast, my mother is currently Principal at an inner city Catholic school and each of their classrooms is half or more non-Catholic.
And Guin’s right, a Non-Catholic could be as discreet as they wanted but it would be obvious at Communion time. No one would care though–hell, IIRC the boys in my class were jealous of the non Catholic kid because he didn’t have to do a rotation serving mass.
:dubious: Do have any idea of what you just implied? How many children do you know who have unconfessed mortal sins?
Any of which would be highly unusual and noticable.
Maybe they just did not have a chance to go to confession before Mass, and they’d rather play it safe. Maybe the kid believes that having used his sister’s Barbies to stage a recreation of a witch burning was mortal sin.
In my Catholic schools it was entirely unremarkable for a bunch of kids to NOT take communion. Catholics are encouraged to have it often, and required to at least once a year during Eastertime, but nothing mandates that they partake every single time. My HS class (some 80-strong) included a number of protestants, one ostensibel atheist and one (1) Jewish transfer student (but then again even in a public school in SJ that last item would be remarkable, due to our demographics). He did not have to attend the Religion classes or chapel. All nominal Christians did, with just noncommunion.
http://www.answers.com/mortal+sin&r=67
“Some sins that orthodox Catholics consider to be mortal (if the conditions above are met) include adultery, murder, lust, missing mass on Sunday (except when one must, e.g., child care, dangerous roads, etc.), perjury, incredulity, and the use of contraceptives. It should not be said that the sin of missing mass on Sunday is considered equal in gravity to murder: the Catholic belief holds that mortal sins can vary in their seriousness, although the “mortal” effect remains present for all sins in this category.”
The answer would be many teenagers. See also:
I’m not in the US, so maybe my experience isn’t the same as for people who studied in the US, but when I was in high school, I went to a Catholic school. I’m baptised Catholic, but I’m now an atheist and I pretty much already was back then. I know that many students weren’t Catholic (a few Protestants, a few Muslims too I think, and we might even have had a Muslim teacher, I’m just not sure), and it never was a big deal, not even a small deal. In fact, I have no idea of the religious affiliation of most of my fellow students from that time since it just wasn’t something people usually talked about. Not any more than in public schools. For our first four years out of five, we had to take Catholic religious education classes, but now that I think about it, they don’t seem to have been particularly “Catholic”: most of the time, they were more like generic Christian religious education. I certainly didn’t hear about transubstantiation, immaculate conception and papal infallability or other things like that until I started to become interested in religions, a few years later. The school didn’t organise Masses; there was a chapel in the school but it had been converted into a small gym. And in our last year, we didn’t even have a true Christian education class: we chose between a moral education class and a class that taught about the beliefs, practices and history of the major world religions. I chose the latter, and I don’t regret it; we had an excellent teacher who might be in part responsible for sparking my interest in religions.
Call me weird, but this reminds me of a joke by stand-up comic Yvon Deschamps. His character is talking about his youth in a strict Catholic family. So he says (I’m paraphrasing): “They say that the flesh is weak. Let me tell you, the flesh is not weak. For a 12 year old boy, the flesh is very hard! We were in danger of damnation every 20 minutes!” 
OK, I guess I don’t tell it as well as he does, but when I heard it it was pretty funny.