>her monolingual English teacher has told us that she is far ahead of the rest of her class language wise //

How do you know what her ability in English would be if she'd not had the other 2 languages. Like I've of the languages could have buoyed it up but the other overloaded her and reduced her overall vocabulary. Or many other possibilities.

Surely the truth is you hypothesise it had no effect but without re-running her life you can't know one way or the other?

What do you mean with your last sentence, it's entirely incoherent to me?