a high school student can and should try to read anything he/she likes. actually high school students may be brighter than college students.
and often more motivated. so plunge right in.
i read principles of mathematics in high school, and it had lots of great book references at the ends of the chapters. then i went to the college library and looked up those books
i still remember sitting in the stacks puzzling over the proof that there exist an infinite number of prime integers.
let p1,...,pn be any finite set of primes. and then consider the integer
N = 1+p1p2...pn, 1 plus the product of all the primes pi.
we claim no pi divides N, because if say p1 did divide N, then since
1 = N - p1p2...pn, p1 would also divide 1, which is false.so none of the primes pi divide N. But N is larger than 1, so we claim some prime must divide N. I.e. among all divisors of N greater than one, there is a smallest one say q.
then q cannot have any factors larger than 1, or they would be smaller factors of N.
so q is a prime factor of N, but q cannot equal any of the pi. so the finite list p1,...pn, is not the full list of all primes.
hence there is an infinite number of primes.