Victor, When ever "classes" is used, I feel that they refer to the "lectures" you took that day, specially if you're a university students. Lectures in Arabic is: محاضرات, It's the plural of one lecture: محاضرة. But if you're still a school students, then I agree with the 2 previous answers that say the word classes stand for:حصصك, which is the plural of one class:حصة. That makes your question in case of a school student: كيف كانت حصصك اليوم؟
And in case of a university student: كيف كانت محاضراتك اليوم؟