I like #2 the most I think. #3 also looks good, but yet has a little "childish" look to it, making it more suitable for a kids' math site. #13 is nice also, but it looks just like 90% of every single "Web 2.0" logo on the internet.
I like #3 and preferably #6, especially if the audience are kids (since you mentioned students). However, if they are older kids, #1, #2 or #15 would be my choice.