Is forgetting a lot normal for a 15 year-old or is it a bad sign?

Mostly normal: Unless there are other signs of brain damage, memory problems are usually a matter of deciding whether you want to remember something in the first place. A 15 yo might be so distracted that they have trouble registering names or other facts when they hear or read them. Many people of all ages don't take the time to commit most facts to memory and then complain that they have bad memories.

No, not normal: A 15 year old who's forgetting a lot needs to see his or her family doctor for an evaluation. It could be not paying attention, adhd, doing drugs, not getting enough sleep, being depressed, or many other things going on. It's not normal.