"I'm in (a) class right now" with or without "a" is better as a general statement, to show you're busy. "I'm in THE class right now" would be if someone had asked you if you were in a specific class right now and this was your response to them. Adding "the" in front of common nouns is almost always only used for when all people in the conversation are aware of the exact subject, so in this situation, I don't know what class you're talking about, so "the" shouldn't be used.

Thank you (__)

you should say “I’m in class right now” unless you have been talking about that class. For example, you are on the phone with your friend, she asks you, "when are you going to be in your English class?, and you say “I’m in THE class right now”

You are in class, but in the classroom. "In class" means attending class. "In the classroom" dessribes where you are sitting. PS: if you aced the last test, you "got the best score in the class" because there "the class" refers to the group of students enrolled in the course.