Based on the page for Poké Pelago on Serebii, it looks like this is just to help random players over the Internet. It won't give you any benefit except the joy of knowing that you helped a stranger.
You can also, once a day, send out a Bean Bottle. This will put 7 of your standard beans out and distribute them across the Internet to other players and you will sometimes find Bean Bottles from other players turn up on your game.
From the same Serebii link as above, there are a few strategies to improve your chance of having a Pokémon call for help:
The first is to simply keep the Pokémon at a low amount of health. A move such as False Swipe will work for this. Here, the Pokémon has a higher chance of calling for help. It won't call for help if it currently has assistance.
Another way is to use the brand new item, the Adrenaline Orb. This item is found in all Pokémon Centers and, when used during a wild battle, will make the likelihood of the Pokémon calling for help even higher. You can only use one per battle, but the effect should last for the duration [of the battle]
Due to Adrenaline Orbs being usable only once per battle, as noted by @UnknownZombie in the comments, using additional Adrenaline Orbs is a very effective way to pass turns during battles where you are trying to chain SOS Pokémon, as you will not be able to use another Adrenaline Orb, but it will use up your turn.
A third way to improve your chaining probability is to use a Pokémon with the ability Intimidate, Unnerve, or Pressure.
The ability of the Pokémon you have in battle is also one that factors into S.O.S. Battles. If your Pokémon has the abilties Intimidate, Pressure or Unnerve then the opposing Pokémon is more likely to call for help.
Thus, the three main strategies are to keep the Pokémon at low health, to use an Adrenaline Orb, and to use a Pokémon with Intimidate, Pressure, or Unnerve.
As an aside:
Do note that even if a Pokémon calls for help, it is not guaranteed that a wild Pokémon will come to its aid.
Best Answer
The best solution would be: don't use an email with recieve restrictions!
If your school email cannot recieve outside email, you're just going to have to set up your account using another email account.
If you're looking to "activate" that means you've not got an account yet, so there's nothing to actually change - just start afresh.