Well he didn't use Sera's abilities so I have the same question!
The reason why John can't use Sera's power is because he didn't activate his power when he face off against her.
My theory is that he can only keep one power at a time and he lose the power he mimic every time he mimic a new one.
However,
If he really can keep those power permanently and switch different skills... holy shit no one could beat him. He's a fuckin cheat with such godly power.
Regardless, I think there's still lots about john power that's still not explain.
One power at a time? But in the scenes we see he uses at least to at the same time. The ones from the girl and boy (whirlwind and claws) AND he uses Arlos barrier too (but not at the same time as the other powers? Not sure)
But i really don't think he can hold them perm. That way he probably wouldn't have used the powers of boy and girl or Arlo but one (or maybe two) of the powers of the students of his old school instead. So Arlo wouldn't know that John can copy powers, but has - I don't know - laser eyes or whatelse xD
Yeah! There is lots to explore! His power level for one. Or the question if his father (or mother) has the same power. But now the book his father wrote makes sense. For John, all the other people are like cripples with their limited powers.
A cheat? No. It's not his fault that he copies other people's powers which is what cheat implies. But I still do like your theory about how he has to activate his powers in order to copy others' powers. It could be applied to all the other powered individuals. I never considered that they would actually have to consciously 'switch on' their powers.
Still firmly believe his ability is above god-tier. No one can convince me otherwise!
On another note, you think how long he can keep the abilities of others? What's his time range? It can't possible be permanent. So it's like a few minutes? A few hours? If we take in the states of those two students I guess John will keep abilities even beyond the death of their original owners.