Have we scheduled a class today? Do you have time for a class today? May I check if we have class today? Will today's class be as scheduled or is there an amendment to the time? That's great, thankyou. [sure] 6pm is great. 6pm works for me. 6pm is perfect, thankyou. See you then!

Here are some possible ways to ask and respond to these questions: Are we having class today? Can you confirm if we have class scheduled for today? Is our class still on for today? Are there any changes to today's class time? Has there been any update or change to the scheduled time for today's class? Do we need to adjust our class time for today? How to agree on a time to have class, other than “sure”? That time works for me. I am available at that time. Yes, let's plan for that time. How to say I'm okay with having class at a particular time, like “6pm is fine”. 6pm works for me. I'm available at 6pm. Yes, I'm okay with having class at 6pm.

"Are we having/Do we have class today?" is fine. "Are there any changes to today's class time?" is fine except I added an apostraphe. "6pm is fine" is fine. "Sure" is just a all-purpose word of agreement. You can almost always substitute it with any of the following: OK, fine, agreed. Or you could be more specific to the statement at hand: "I'll be there."