Stacy and she are going to the gym. Since the class starts at one, they are taking the afternoon bus.

Is this a valid sentence?

well, it looks like two syntactically correct sentences to me.

Well, this sentence is valid grammatically speaking, but I'm not sure about Stacy and she taking the bus in the afternoon. I hope they're not too tired from their workout to stay awake on the bus!

Yes, this is a valid sentence.

Yes, this is a valid sentence. It is grammatically correct and conveys a clear meaning. The sentence states that Stacy and another person, referred to as "she," are going to the gym. It also mentions that they are taking the afternoon bus because the class starts at one. Overall, the sentence is structured properly and makes sense.