Don't know what country that is, but most in the US are not 18 when they head off to be freshmen to college, if so, they are just turning 18 when they enter college. I know when I graduated ages and ages ago, there was less than 10 out of 100's that were 18 and that was only because my parents wanted me to be "older" when I graduated. The perk of that meant I was one of 5 freshmen in high school that was old enough to drive
It depends on state and time period, but these days most college freshmen are 18 by September. Unless they skipped a year, they should all be 18 by the second semester, but looking at some stats from 18 years ago, at least 2/3 of students are in a state where they have to be 5 in september the year they start kindergarten (which with K and 1-12, leaves you at 18 the September you are going into college). Some colleges start in august, many start in september.
If I could fully nerd out for a moment, that leaves less than 1/3 of students being from states where they could still be 17 by the end of September in their first semester of college. Assuming they are all turning 18 by the end of the calendar year, that means only 1/3 to 1/4 of said students would be born in the later months of that year.
You end up with 10% of American college students that could be 17 after a month or so of college.
edit: Went back over the comment thread and realized this was originally talking about High School seniors. In that case, 40-75% of students would be would be 18 by graduation, likely much closer to 75%. Virtually no one would be 18 at the start of senior year, unless they were started attending school later or had to repeat a year.
tl;dr: College = probably safe and legal. High School = not safe and likely illegal.
