Is it obligatory to donate a sum of money (around one hundred pounds) that was not paid to private tutors ten years ago, due to a feeling of not having fulfilled their right?

You must take the initiative to return the rights to their owners if you know them. If you despair of reaching them, then give charity on their behalf. If you find them, give them the choice between the reward of charity or the restitution of their right, and the reward of charity will be yours. Your feeling that they were negligent in the lessons does not permit you to take their right; the consideration is the agreed-upon contract.