Max is younger than Justin by about 3 years(In season three, Justin is a Senior and Max was once referred to as 14) so whenever it is time for the Wizard Competition, wouldn't he have a disadvantage since Justin would have had more time to learn magic than Max? Discounting the fact that Justin is the smartest and whatnot, I would say they might start training at around the same age, maybe at 6 or 7 or something. In total, Justin would have the most years to study for the competition, and the fact that he has been in one already gives him a even huger advantage over Max...Yea, makes sense?
Yes, and this is an excellent question. Unfortunately, the only answer I can give is "The wizarding competition is inherently unfair." Think about this – if after the kids go through the competition, Theresa and Jerry have another kid, then presumably, they won't even have a chance at the powers. (Or maybe the council would allow for a retest, but even then, his siblings would have had much longer to train.) The council doesn't care – whatever their reasons for levying the competition are (likely to keep the number of wizards down), justice isn't one of them. Practicality is.
I, for one, assume the competition occurs when the eldest reaches the age of majority, which in the wizard world, could be anywhere from 18 to 21. Just a guess though.
If Alex doesn't test well and is a year behind, and in the same grade as Harper, does that mean that Harper is a year younger than Alex?
Possibly. On the other hand, it's possible Alex was a sophomore with Harper in season 1, but had to repeat the grade in season 2. So Harper would be a junior while Alex was a sophomore.
When does the movie take place? In Justin's New Girlfriend, Alex's hair is short, but in the movie it's long again, but they make it seem like the movie takes place right after Retest.
I believe it is supposed to take place after Retest, but it might not. It might take place right after the Vampire Saga. Since we don't know the exact amount of time elapsed, it's possible Alex's hair grew. (It's also possible she can use a spell to style it however she pleases at any given time, of course!)