What is the back to school promo? Is it like a discount?
Also do you have to be a senior(highschool) or a college student? I am a junior in highschool and going to buy a Macbook Pro next weekend.
Back to School promo is something Apple runs every year which is
in addition to the education discount, which runs all year long.
The education discount is only for higher education, but also includes high school seniors going to college next year. Normally you would not qualify, but it's worth a shot asking anyway. Considering that it's like a $300-$400 discount when you add up the price of the Back to School promo, plus the normal ~7-10% discount on the price of the computer itself, if they won't give you the discount directly it might be worth asking one of your friend's who's a year older to go into the Apple store and buy it 'together' with them. (Note that you can only buy one type of computer per year with this promo, so if they're also planning on getting a Macbook Pro, you can't both do it, but if they want an iMac and you a Macbook, or if they don't want a computer at all, then it works.)
For the OP, as for dates: take a look at the history of the promos
https://www.tekrevue.com/apple-back-to-school-history/
Which misses the last two years:
2015: Started July 23
2016: Started June 2
So, the earliest is 25 May and the latest is 23 July. Best guess is: it'll start sometime between June 12 and July 31st. Since they just released new macs, I would guess they will start somewhat towards the later end of the summer this year in order to punish extremely impatient students, but you never know.