It's better not to use the definite article for 'break', the same as we don't say 'I go to the lunch at 3pm", but "I go to lunch at 3pm". Same for breakfast, dinner etc. Also because 'break' is a time of the day, it's more natural to say something like "break is the best time during my school day. At that time we go to...." English often has to be quite specific :) And it's best to put the location (i.e. "canteen") before the verb "to eat", because you have made this the subject of your sentence by saying "we go to..." so... "Break is the best time of day during school. At that time we go to the canteen to have our lunch. After break we go back to class. In the evening we have to stay in school for practical classes." But it all sounds great and definitely makes sense!

It is fine.It just means "the best time, or thing" but the word "time" or "thing" has been omitted, because it is understood.