Your life right now seems pretty hard. I'm very glad you've come on here for some help and someone to listen. There are plenty of people on here that want you to feel better. School sounds really hard to get through. When you don't have people to sit with at breaks or in class you probably feel more alone. I understand how you feel when you look at others and wish you were like them. It's a hard emotion to deal with.
What your teacher said about putting a happy mask on, I really don't think its a good idea. It might help you seem more approachable or joyful, but ignoring and pushing away your emotions is not going to make you feel any better.
You are not destroyed, unfixable, broken or lifeless. You are worthy. You deserve to feel happy. You are important. Death isn't a holiday at the beach, its a permanent thing, impossible to undo and not enjoyable for anyone at all.
Are you receiving any professional help? If not, reaching out to someone is a good first step. Kids Helpline (1800 55 1800) are really good listeners and Lifeline (13 11 14) are also great. Are you willing to talk to your family or any other teachers/counsellors at school?
Your life sounds pretty tough but just know there are brighter hopes for the future. Things can and will get better.
Hope some of this helps,