Since I bought my BMW in 2014. I have always had ...

Since I bought my BMW in 2014. I have always had an issue when I have taken it in for routine maintenance. 3 weeks ago I had it in for oil change and a few days later the check engine light came on. My husband looked under the hood and found that they had not hooked the ducting to the air cleaner. This allowing the engine to injest the air directly from our dirt road. My husband called the GM at Schomp and we were told that college students are being used to do oil changes on our vehicles. They are supervised by a certified mechanic. Obviously the certified mechanic was not doing his job on my vehicle. They advertise certified mechanics work on the cars. What else was overlooked? I've lost my confidence in their ability to perform quality service on my vehicle.