In my experience, both are used.
Student or Learner
For this, is it better to say
1- Three plus three equals six.
2- Three plus three is six.
I know both are right but which one is used at schools.
Both are fine, but I think is would be more common in BrE.
I think "is" is more common here too.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
Thank you very much.