Praise be to Allah.
What is mentioned in the question is a very real dilemma, the matter of protecting children’s hearts and minds against the ideas, beliefs and theories they will encounter in the school curriculum that are contrary to the basic principles of Islam. Protecting the children’s morals, attitudes and behaviour, despite what they are exposed to in mixed schools that is contrary to all of that, is a dilemma; it is the dilemma of confronting specious arguments and whims and desires at the same time. What seems to be the case here is that it is the father’s duty to choose for his son the best school and the one that is farthest removed from these corruptions. If it is difficult to find something purely good in one place, then he should look for something that is most good and less harmful and evil.
What you have to do for your son is to choose an Islamic school for him that is not mixed, even if that costs more in school fees or involves greater effort in moving house to find the school. Indeed, we advise you, if there is no such school in the city where you live, to move the family to another location where you will be able to find a suitable Islamic school for your children, if that is possible for you, even if it involves some hardship for you.
If that is not possible, then you can replace these schools with education at home (homeschooling), even if the standard of education is slightly lower. So go ahead with that, and give precedence to protecting your children’s religious commitment and morals over everything else.
If you are not able to do that, and you have no choice but to put your son in these schools, then you will have the burden of following up on your son, watching out for him, and resisting the causes of corruption that he will encounter.
If you are saved from it, you will have been saved from something major.
But we think that it is very difficult to make this choice and continue doing it as it should be done. Hence the options mentioned above may be more realistic and logical.
See also the answer to question no. 127946
We ask Allah to guide you and help you to shoulder this responsibility.
And Allah knows best.