I'ld like to claim a case against tutoring center for not achieve estimated achievement. Whom shall I contact

My son had been tutored by one of the famous tutoring center. Before that they ed him and estimated his hours to improve his comprehension level.. They estimated, 56 hours will take to improve his level into his grade level (by that time his comprehension level was blow his grade level), 110 hours to improve one level ahead. After 40 hrs completion(4 months later) they took essment test, now he has improved only 5%(still he is in below his grade level), he supposed to be improved 80%. We would like to claim a case against them…If it is positive, whom shall I contact for that?