School counsellors can be helpful, but they tend to deal with less complicated issues. MH support via your GP (either in IAPT or in secondary/tertiary care) tends to be for people with greater need or poor access to things like counselling. You could try your school counsellor first, and see if it helps? If it doesn't, speaking to your GP is a good idea too.
In the mean time, try and make sure you're taking care of your body and do what you can to support yourself in your mental health. Speak with people you trust about how they can support you as well.