1. Students should concentrate on their studies.
In the case above concentrate is the verb and tells what the student is doing.
2. Students should be concentrated on their studies.
The sentence above does not make sense to me but i think intent is to modify students. Thus, concentrated is an adjective in this case.
3. Most parents are worried about their kids.
In the case above worried modifies parents as an adjective.
4. Most parents worry about their kids.
In the case above worry is a verb and tells what the parents are doing.