Car pulls from side to side at times

Sometimes can't drive straight will have to try to correct steering wheel to drive in a line. Our daughter is 15 just learning how to drive so we brought her a 2000 ford focus wagon with 164000 miles on it to patter around. She only goes to school and lessons. The car was $1500 and we took it to midas to get it checked before purchase. Midas said it was a good car and only needed $80 worth of repair for a missing oil cap they also said the tires were bald in the back and replaced the two back tires total came to $383.

We took the car in to get this done after the purchase but at times we found the car will pull from side to side. You will be driving and have to turn the wheel right and left to drive in a straight line. We took the car back to midas but they said it hasn't happened when they drove it. We have not let our daughter drive it because we are afraid it will pull to the right or to the left and she won't be able to control it. What in the world is happening here? My husband says it might need alignment but I am afraid it still might not do it and we will have paid more for repairs than for the car soon. What is causing the car to go from side to side sometime when it's being driven. The air also is always on we live in florida. Thank you for preventing our daughter from getting into a accident.