It's time to have a class.还是It's time to have class.

it's time to have class.上课时间到了.have class 一般性概念,表示上课,有课,不加冠词.又如：
do you have class this afternoon?今天下午有课吗?
若表示具体的某堂课,具体的.课,加冠词.如：i had a Chinese class just now.
I am having an English class.