Is a 25 year old guy too old for an 18 year old girl?

Well my friend and I are seniors in high school and we graduate in like 3 weeks. There is this substitute teacher and he is a cutie. My friend has like the biggest crush on him. They started talking last year and now they are dating. My opinion is that age is just a number, but the thing is she doesnt want anyone to find out at school until we graduate. Also you see celebrities with greater age differences. But we all dont think the age is the problem. If he is 25 and she is 18 is that too big of an age difference?