втыкать на парах
втыкать
literal: to make something thin, or something with a sharp point or just sharp plunge into something solid; Он воткнул нож в доску - he plunged the knife into the plank
slang: to understand something (I think usually in questions and negatives): Слышь, ты чё вообще не втыкаешь, чё тут происходит? - red neck use
slang: when your thoughts are being somewhere else, so that you no longer pay attention to what's going on around you; (the defenition for your case); Он долго втыкал, затем резко повернулся и спросил: "А, о чём мы только что говорили?"
slang: to look at one spot for a long time because you're bored, falling asleep etc; to stare; to gawk. Он долго втыкал в граффити на стене, пытаясь разобрать, что они значат
slang: to beat someone up, especially with a movement that reminds of that discribed in literal sense; Да, я тебя щас воткну тут! - red neck use

пара
a couple - он и она - пара - they're a couple
a pair - пара ботинок - a pair of boots
one particular class as in "I missed biology class" - we say пара because that one class is made of two academic hours, so that's a couple of hours. That's true only for Universities, Colleges and so on, not for school. because in school it's a one academic hour per class, so it's урок in schools