Responses
I would guess probably around at least 8.5 to 9 if not 10 (or even higher if that's possible)
But additional to the hints you posted we also know that John at his old school was Rank 1 aka King. So his power should be pretty damn high.
Also we now know that his power really does something to the ability of others (since it was stated that you shouldn't use your ability while handling John)
Guess what's John's Level??
Chapter 47. Page 47-57
Hint: "HOLY SH*T!" "That Level..."
Hint: "...explains how he got into Wellston" - For people with powers (duhh... But we all know that he does have powers and we're really curious what it is. (づ￣ ³￣)づ)
Hint: Has unparalleled confidence. - Very strong, confident, no fear (his past was a definite fear and hate o-o), a bit spoiled. lol.
Hint: "And how could any school even think of expelling him?"
"With powers like that" - Very high power maybe even higher than Seraphina and the Blonde blue eyed (looks like a foreign) guy...
Hint: Expelled from New Bostin HS because of excessive violence
Hint: Isen is shooked by saying "WHAT THE HELL?!" because of "single-handedly took down half his class...for an unexplained reason?!" (but we know the reason ppl)
Hint: "He's not someone to mess with!" "If we expose him, who knows what he'll do?!" means first, his like the old Seraphina that you shouldn't mess with, second they won't expect what will John do to them... (of course because they don't know why and what his eyes glinted).
There's more hints in the manga itself but too lazy...