One thing, is that yes by going to some school related source you would be going to a therapist. A therapist could be a really good option for you, believe it or not. They are legally confidential - they have to keep your secrets - except in some cases. If they know you're in physical danger they can break confidence, but in the case of SI cases are highly situational.
If you really show that you want to get better theres a good chance that the therapist can help you, I just want you to check out that option. Some people get in 'bad' situations by going to school, I'm sorry about your friend.

One way, is if you ever felt like talking to your mother is ok, you could ask her if going to an outside therapist was an option. That way, it's not related to the school, it's a better place to be really.

the difference between stress and depression, can be a bit difficult to tell sometimes.

Depression is "deeper" than stress.
They both affect each other.
I guess the difference I'd put is that stress is passing, depression stays with you longer. Maybe somoene can phrase/explain/give better answer than that, I'm a bit tired