Understanding the context:
* Are you asking them something?
* If so, you could say: "Ah, vale. Entonces, ¿te puedo hablar más tarde?" (Alright, then. Can I talk to you later?)
* Are they telling you they have to leave?
* You could say: "Vale, que te vaya bien en clase" (Okay, good luck in class) or "Hasta luego, entonces" (See you later, then).
* Are you in a casual conversation?
* You could simply say: "Ah, vale" (Alright) or "Que te vaya bien" (Have a good one).
Other options:
* "¿Qué clase?" (What class?) - This is a good way to show interest and keep the conversation going.
* "¿De qué es?" (What is it about?) - This is similar to the above, but focuses on the subject of the class.
* "Ya te hablaré después" (I'll talk to you later)" - This is a good response if you don't have time to chat right now.
Important note: The best response will depend on the specific situation and your relationship with the person. Use your best judgment!