July, August and September are the highest birth months in the USA over the last 30 years....so statistically, the likelihood of being in 'high' school and being 18 is very slim. Given generally high school finishes in may/June, too far out of the slim window to be a8 and in high school..you'd either be in the year below or already finished and left as you'd have been in the year above.
All these adult games where every single girl (and the mc) are 18 and still in school is quite frankly, ridiculous.