Respectfully, perhaps those three kids who returned home lacked self-discipline. CU-Boulder is no more of a party school than any other large flagship. Nobody is forcing kids to party. Students are free to choose friends who share their values and/or live in dorms not known for large parties; both the honors dorm and the E-Quad dorms are quieter.
I truly believe there is a place for everyone at CU-Boulder. I entered CU-Boulder as a low-income student and found my people there. I encourage you to visit and decide for yourself whether it could be a good fit for your child.