High school is around 12,000bht/year, if you don't pay the 'extras' they don't accept your kid.
University loans are hard to get, and then need to be repaid.
Lets assume you borrowed 30k/year, outstanding loan 120kbht.
Then the job you get pays 20k/month, and you would have earned 15k/month in a supermarket without a degree.
How many months do you need to work to break even?
720k loss from not working 4 years + 120k debt = 840kbht loss ..... made up by 5kbht/month wage increase.
So that's another 14 years to break even with someone that left after high school age 18.
or another 18 years to break even with someone who left school age 15.
Seems you don't have much experience of actually living in Thailand.