1. "Can you buy milk?" is more accurate, though "You can buy milk?", can be used to ask if milk is able to be bought there, or as a suggestion to someone to buy milk, though this is harder to explain through text. Depending on what you want to mean.
2. "Can you buy chocolate?" sounds more natural.
3. "I don't have to go to school." "It is summer now". sounds more natural.
You sound great and most English native speakers don't know how to properly write from what I've seen, so keep up the good work! :D