yes
You should give Mindef CMPB a call to confirm that they will allow you to defer NS as a private candidate (because they usually require the student to be enrolled in a recognized school for deferment purposes).
As to whether you can get good grades as a private candidate with private tuition, the answer is of course it's possible, as long as you're willing to study hard.
Self study as a private candidate will not get you a NS deferment.