Do things to cheer you up, maybe take up a new hobby, it both gives you new experiences and also let's you socialize with a new group of people, watch a good film or listen to music.
Another thing you could try is writing it all down, how you feel and what things that has happened that makes you feel like this. If it's your grades, then make a decision to improve them. Try joining a study group or get someone to help you, set goals for yourself and award yourself when you've accomplished them. Also, try reaching out to some new people, wether it' someone in your class or maybe online who you could talk to as well when you feel like it.
Also, I found this article about 5 ways to stay happy; Stop chasing and start living,Wanting more is fine, but don’t forget to relax and enjoy life for what it is – in the present. Assume Responsibility, Instead of blaming others for what’s wrong in your life, focus on what you can do to make things better. Stop Seeking Stimulation, If you want to be happy, overcome this addiction. Develop the ability to enjoy life in its entirety – even when the stimulation stops. Enjoying every experience will give you many new reasons to be happy. Take Action , Educate yourself and commit to find happiness no matter what it takes. With enough hard work and dedication, you really can create the life you want. Expect Nothing , Accept life in its entirety; stop thinking in terms of what should be and accept what is. When you live without entitlement, every good thing becomes a wonderful surprise. Even better, expecting nothing means never being disappointed.
Finally, I'd like to leave you with some words from one of the greatest minds; ”Happiness is when what you think, what you say, and what you do are in harmony.” – Mahatma Gandhi