"Коли в тебе завтра перша пара?" would sound more natural, in my view.
Is пара standard here? Would клас also work? (of course also changing the grammar)
A Pair of Shoes???
What about a 'lecture', a 'class', or a 'lab'???
Assuming the etymology is similar to the equivalent term in Russian, it originates from university lectures having been two hours long.
"Period" should also be accepted along with "class" as a correct translation for "пара", shouldn't it???
I think "period" tends to be a high school / middle school term for "class," and I'm thinking "пара" is a college/university term.