Students mustn't smoke or drink while＿＿ at school.

B
while后面只能加进行时
而且此句是一个省略
应该是 Students mustn't smoke or drink while they are studying at school.

B。
Students mustn't smoke or drink while studying at school
当学生们在校学习期间不允许抽烟或喝酒

由while引导的时间状语从句 省略主语 采用动名词形式做伴随状语

B,主语一致可以省略