Beginning with this post, I am going to continue the advice to my grandsons, but I will also add a grammar or usage tip that they or anyone else might find useful in their writing or speaking. Notice that I said tips and not rules. (Rules can be long and confusing, and in English are made to be broken.)
Always read a contraction as if it were two words to be sure that you have chosen the correct spelling.
Choose the correct word: (Your, You’re) going to be late to class.
Read the contraction as two words. “You are” going to be late to class. Now it is easy to see that the contraction is the correct choice in this sentence.