a. he asked this during school hours
b. he asked it in the capacity of a public school teacher

he was advertising a religious activity when he was being paid as a teacher

it would be different if he ran into the kids at a bar (oh right 17y olds can't drink in the US :) and asked them, but even then it's highly fishy because kids could feel presurred because they feel it might affect their grade or position in the team

and as Cenk said: what if it was a mosque or a satanic church

+ why the F would you want to go baptize a bunch of kids, the fact that he wanted to do this  should trigger an investigation into this guy