Grounding a teen is an individual choice and also depends on the teen. If they are trying but still struggling then they may need extra help such as a private tutor to catch them up in school. You may also need to home-school if they are getting in trouble at school.
Counseling may also be an option if failing is new experience and see if they are having emotional difficulties.
I think the school teachers and counselor would have made attempts to contact the parent before now.
Working together for a solution is the best approach.