If the school has an unprotected network, which most of them don't, then a person could simply visit the website of the BTD games and play them. However, schools often don't allow students to play games on their computers and the student will likely be punished for doing so.
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Try coolmath games for kids it comes up with btd
Use cheats
Only if the school had not already blocked that website
They don't need to bring them to school. School is to learn and the students would rather play games. Some games are very violent and sexual and these don't have a place in school. Finally, the games are expensive and could be damaged or stolen.
Hooda math is the only game I can think of.