You still don’t understand. After a couple of years the system will stabilize and each year most of the newest boys will marry, in addition to diminishing numbers of 2nd and 3rd year boys and nearly all the boys who ever marry are married after their third year. Therefore there will never build up significant numbers of very old boys. However the girls only have one year to get married.
The “prime” members of the new crop of sem girls returning each summer are snatched up.
Obviously this is a limited and inexact model, but is much more accurate then your static one.