Why do I find it hard to concentrate?
When you grieve your brain is overwhelmed by so many different thoughts, feelings and emotions there is little room left for anything else. Grief takes a lot of energy. As a teenager your brain is still developing and you have fewer coping strategies than adults, so you can often feel overwhelmed by life; maintaining grades, socialising, activities, a job… This is normal, you are not going mad. You might find it helpful to look at the impact of teenage brain to cope with bereavement.
If you are finding the demands of life challenging make sure you take steps to be kind to yourself, assess how and where you can minimise the expectations on you – what can you temporarily cut back on? Ask for help and support from your teachers and school, do they have a counsellor or someone you can talk to?