The answer is tighten and contract.
In order for the students to pull the rope their muscles will need to tighten and contract so they are able to continuously pull with all their strength and power. Relaxed or twitched muscles will result in release of the rope.
To demonstrate responsibility, the character must show initiative to do things that aren't necessarily just for fun or for your own benefit, such as helping a charity or cleaning a the house. The character will be able to look after others and show real selflessness.
