is it legal for school to jam cellphone service?

If it's not legal it should be.
Schools are no place for a cell phone. You're there to learn not to pfaff about on your i-Thingy.
They're not allowed in a lot of workplaces so why should schools be any different.

Absolutely. It's also legal to ban the use of mobile/ cell phones in school.

who knows. all i know is that you should be texting less and learning more

No, you are in school to learn, not to goof off and play on the phone.

Yws