Depends on how many lessons (classes) you have. In general, the -es affix makes the plural form. Example: One lesson = class Two lessons = classes However, I should note that both forms are correct in the two sentences you've noted, at least in everday speech (if you were discussing your schedule with someone). 'I have class' is more colloquial (i.e. between two students). Here it comes down to context and who you're speaking to. You might want to check this out: https://forum.wordreference.com/threads/have-class-classes.2894569/

In American English the sentences are equivalent, in British English we would say 'classes' or 'school'.