Can a parent of an 18 year old get a restraining order against his girlfriend because they don't like her? Can a parent of an 18 year old force him to change high schools to keep him away from his girlfriend?

NO one can get a restraining order against ANYONE just because they don't like them. They would have to prove the person is a danger to them.

And no, you can't force him to change schools. But you may be able to force him to move out if he doesn't comply.

But frankly, the more you try to come between them, the more likely you are to drive them together.

No, a restraining order requires a serious threat or danger.

You may be able to change his high school, but it won't help,

I agree with the others and add this: think about what this will do to your relationship with your son.

What are the reasons? Is there an immediate danger? Is pregnancy eminent or is she influencing him to use drugs?

It is hard to know what you are avoiding from their relationship. I would at the minimum make sure he has access to contraceptives, pref with nox9 and a thin barrier so he isn't tempted to go without.

I would say I agree with ScotGem, you will drive them closer. Kids love a Romeo+Juilette martyr scenario to play at. Your best stance is to try and influence him in a positive way until the relationship plays its course.