verb + s in this sentence?

[ysc1230]

She insists that her son attend private schools to ensure getting the best education possible.

I am not sure why we does not need to add "s" to the word "attend" .

If I rewrite this sentence, I would add "must" or "should" before "attend".

Thank you for your help.

[5jj]

The third person form with -s is the only recognisable present subjunctive form for most verbs. It is frequently used in AmE after such verbs as insist, recommend, suggest.

Most speakers of BrE today use either the normal, indicative form of the verb, or should:

She insists that her son attends/should attend ...

[ysc1230]

Thank you for your prompt reply

Which form should I use in exam? e.g. IELTS written test.

Would the maker consider both are correct and up to the student?

[5jj]

I am fairly sure that Cambridge examination markers would accept, but not expect, a subjunctive here.