It is a matter of probability. Males produce 1/2 sperm cells carrying the X chromosome and 1/2 with the Y chromosome, thus determining 1/2 girls and 1/2 boys in the progeny, respectively.
If different matings are independent (and this assumption could be analysed separately for matings within a couple and among couples), then the probability of having a progeny or generation of n individuals of only one sex is 1/2 to the power of n.
In your question, n is 5 or 500, then the probability under these assumptions is 1/32 = 3.125% (unlikely, not impossible) for the first and a value under 10 to the power of -150, that is, 0.(150 zeros)3, for the second -- virtually 0.
Another assumption that must be made is that public schools draw randomly from the general population, or if there is a bias (for example economical) this does not affect the girls/boys natural distribution.
One assumption that needs to be made further is whether the 1:1 distribution holds from meiosis to conjugation, prenatal development, birth and childhood. It is here that it is believed that some bias is introduced. Nevertheless, all other assumptions being true, if girls going to a public school are 52% of the total, then the above probability would go to aproximately 0.(141 zeros)1.
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