It is best to remember how much suffering you were caused. If their actions against you have caused considerable mental hardship for 7 years now, then repayment must be comensurate.
A lot also depends on how far you are willing to go. If you can't put a price on seven years of abuse during your formative years and the effect this has on the rest of your life, then the sky is the limit.
It is best to be remorseless about anyone who feels your wrath. If you have even a single regret about someone caught in the crossfire, then perhaps you need to focus on individual missions, not group revenge. You may want to organise 'chance' meetings with some of the targets. Be polite and unfazed by their presence. Show a genuine interest in them. Give them a chance to apologise, or at least recognise the folly of their youthful actions. An apology can go a long way, and a genuine apology can heal a lot more than payback. If they are indifferent or wish to re-ignite the flame of burnt pasts, your resolve will be strengthened and revenge will be sweeter.
Considering you only get one childhood, and these people marred your one and only childhood, you need to focus your revenge on something they only get once in their lifetimes.
Graduation, a once in a lifetime event, seems perfect. Next to graduation, you may only get chances like this at births, weddings, barmitzvah's, college graduation, etc...
For maximum effect, you must ensure that whatever you do to ruin this day for them has maximum impact, and thus lasting consequences. Your goal should be to completely ruin this momentous occasion for them.
Public embarassment causes long-lasting emotional trauma, and as an extra incentive it could possibly cause, or start the ball rolling towards complex mental issues and psychological instability for your victims.
You will need to think through your plan, and adapt and cover for every contingency you can think of. One of the worst things that can happen is if your vengeance backfires. This can range from you being identified as the perpetrator (and the resulting legal ramifications), to everybody making the best of a bad situation (your revenge) and making it into something pleasantly memorable, ie; You creampie two jocks and a cheerleader, but they start laughing and turn it into a fun food fight. That would suck major ass because you made it more memorable for the wrong reasons.
To prevent reversals, think ahead to possible outcomes and be prepared. In the creampie example above, replacing the cream with horse, dog, or human shit would negate the possibilities of any unexpected frivolities.
If you have the resources, Private Investigator's or stealth spying and surveillance equipment both make good investments. Each of these will reward you with insights and possibly evidence about what your targets think and do. Weird or humourous sexual activity of your enemies broadcast to those nearest and dearest to them would surely make for satisfying viewing.
Of course, covert recording and exposure of private behaviour need not be voyeuristic if it were to, say, expose mistreatment of a minor, or animal. In those cases, you would be doing the right thing, which highlights the importance of digging up genuine dirt on your adversaries. Genuine dirt is pure gold, and can be used to manipulate the target into situations or actions you desire, while remaining safe and anonymous from a distance. Having dirt on multiple targets is where it gets interesting, as you can pose as one of the targets rorting one of the other targets, and then your assumed identity can be 'accidentally' exposed. This will pit one target against the other, and you can watch as they rip each other apart. This works particularly well if they have been friends for a long time, or are still friends now. The best is if they are related or in a relationship.
Imagine the quandaries if you managed to ruin a relationship, then waited for the split couple to have new relationships or perhaps even children, then provided each of them with clear details on how you manipulated them and directly caused their break-up.
And then link those details back to, say, their families, or a gang or policeperson. If the blame is linked back to gangs or police, then the victims will feel frustration at not readily being able to do anything about it. Or better yet, they decide to get revenge upon a gang affiliate or police officer, inviting their own demise.
But back to your immediate options. Graduation.
Pay some bums/addicts/kids to crash the graduation ceremonies and literally hiff bags of shit everywhere.
Plant some plants in their backyards and then tipoff thugs and police at the same time.
Acting as one of your targets, leave suicide notes/make a suicide call stating you are going to 'take care of everyone' at graduation.
Do whatever it takes to mess up the proceedings at their graduation. If it involves police or feces it is sure to work. If graduation is held indoors, of course the only thing to do is set off the fire alarm, and thus sprinklers. Repeat as necessary.
You do not want this to be a happy day for them.
Remember, it's better to have someone do things for you, rather than do it yourself. Plan it through, and pay the right people.
A pie in the face or somesuch other prank will give only brief satisfaction.
A well thought out plan, as well as a well thought out plan B, will cause years of problems for your targets, and give you years of satisfactory and long awaited justice.