Will a t shirt protect your upper body from jelly fish stings?I am not thinking necessarily the huge box jelly but the more normal types you might meet during snorkeling. I generally wear one to prevent sunburn.
Does it cover your arms? My understanding is that something as thin as pantyhose can protect from the stings, but only what it actually covers, of course.
No but it covers more than not wearing it. Last night the Discovery Channel showed scientists hunting for jellies. They were wearing complete wet suits. But thats in Australia where everything can kill you.
Now I wouldn’t quite say everything can kill you here, probably say most things. 
I saw that same show myself, and despite growing up in North Queensland, (very near to where those scientists where diving), and living with the stingers in the water every summer, I still learnt some new stuff from the show.
In answer to the OP, depending upon the t-shirt - it might help, but probably not. If it’s just a light-weight cotton shirt, it won’t really do anything for you. And in fact could hurt, if a jellyfish, or some of it’s tentactles gets tangled up in the folds of the shirt.
If you’re reasonably expecting to encounter jellyfish where you’re snorkleing I definitely recommend a stinger suit. Which is very like a SCUBA wetsuit.
Generally no a t-shirt is not much help, and the small ones can be dangerous/extremely painful too, depending on where you’re diving. Irukandji are very small but known to cause fatalities, and I was hospitalised as a 8 year old kid from a small portugeuse man of war while wearing a top at the time, it wrapped around my leg and arm. It uh hurt.
Sample cite:
http://www.bugbog.com/travel_safety/dangerous_animals/jellyfish_stings.html
Stinger suit/wetsuit with hood etc and/or avoiding areas/season is the main solution.
Otara
Where are you snorkeling? The only place I have ever been where there was a concern about jellyfish was Queensland. When diving the Great Barrier Reef and the Whitsundays, we were required to wear a stinger suit. In Thailand, Indonesia, Viet Nam, South Africa, Mexico, Hawaii, Bermuda, Fiji, and California, it was never a concern. The biggest danger was a gnarley sunburn on your back, which is why I always wear a T-shirt.
There are box jellyfish throughout the northern Indian Ocean, particularly around the southern tip of the arabian peninsula.
Learn that the hard way 