With regards to university admission numbers, as someone who works at a high level government university, I don't believe that the numbers in Thailand will decrease too much.
The big decrease will occur in the numbers of Thais going to universities in the UK, USA, Australia and other countries. They will have less money and have the risk of CoVid as a good excuse, so Thai universities could benefit this year.
As Chinese universities are oversubscribed, more Chinese students are also choosing Thailand. My workplace has certainly seen an increase in applications. Some private universities - those providing weekend and evening courses - may suffer as their students are at the bottom end and maybe won't have funds for tuition. However, a number of private universities might pick up students who move down from slightly better institutes.
Fewer Thai children are going to schools overseas, but some do and more of this group will stay in Thailand, which will filter down the ladder. Maybe some of the "the owner once went on holiday there" international schools or private schools in tourist areas will suffer, but overall I think that numbers will be dissimilar.
Also, I think the potential decrease in teachers will match the reduction in schools. Many teachers went home for natural reasons and many more because of the CoVid situation. With no new teachers arriving before July, teachers already in the country will see more and more opportunities. I work online and my hours have increased 6-fold since January. This might be reduced as the Chinese go back to school but if you have experience and are being badly treated by your agency then there are many more agencies and a seemingly endless supply of kids.
If you are negative, negativity will find you. Be positive, look for the opportunities.