Is it illegal to date someone in high school if they are 18?

Age, not education, rules the law of consent. If you are both over that age, then it is immaterial what grades each person is in.

Can you date your student if they are 18?

The legal age of majority in most states is 18. It wouldnt appear to be illegal for you to date a teacher in another school district. However, the teacher may have contractual and/or ethical limitations on dating high school students both

Can a 17 year old girl date a 14-year-old boy?

There is no law (generally) against any two people being friends. Intimate sexual activity with minors is illegal. The gender of the two minors is irrelevant. Parental approval is also does not override laws regarding age of consent.