I'm on a trip right now but I will attend the class online.

[alpacinou]

Is it correct to say I will attend a class online?

Is this okay?

I'm on a trip right now but I will attend the class online.

 

[emsr2d2]

If other people are attending in person and you're making it clear that you're going to attend a different way, yes.
If the class is being held only online and you're going to attend, no. In that case, you'd say "I'll attend the online class".