1. What ever you do, don't tell her you are snooping. You don't want to loose her trust or your edge.
2. Talk to her about everything sex related.
3. She is immature, it comes with the age but that doesn't mean her hormones won't try to drive her to having sex.
4. We are more than just hormonally driven and can make choices about sex. Your role is to steer her to a more positive decision regarding sex and waiting as long as possible to have it.
The previous writer spoke of getting a bad reputation from having sex but that is the least of her worries. Point out to her the assorted possibilities of sexually transmitted diseases, the emotional baggage that comes with having sex outside of marriage, the physical toll of pregnancy and/or abortion and a host of other things. Just don't lecture but dialog. Find out her feelings. She may be interested in having sex out of curiousity but hooked is hooked. Once you open that barrel of monkeys it is very difficult to put the lid back on it and you never get all of the monkeys back in the barrel.
It is important for your husband to talk with her too. He is a guy and knows how guys think and why they do what they do. It is a team effort but you must open up the dialog quick fast and in a hurry. Also note she is learning by watching the two of you. Some things are taught and some things are caught. If her self esteem is high it is less likely she will turn towards sex. Build her up in her strenghts and with positive friendships.
I wouldn't change schools because it would be better for her to learn how to deal with boys under your protective umbrella than to begin to learn away at college outside of your comfort and protection. Any way you look at it she needs to learn.