At my school we have affirmation cards that we send to our peers, admin, and students. These are cards (could be any piece of paper or something you design) that one person writes to another affirming something they did. For example, "I really appreciate how you helped me with ____ project yesterday. I was having problems understanding it and you made so clear." ....or perhaps.... "Thank you for your patience when I was trying to figure out my anuncio. I appreciate how you asked me questions to guide me to the answer."
You can give them starter phrases the first few times. After that it will become more natural.
Thank you for...
I appreciate that...
You really made my day when...
It was a cool thing that you....
I have found that even the most hard-core kids will cherish these cards because they don't feel like they are wanted/needed/belong/care. You do have to emphasize sincerity. You may want to filter them before they send them along as a few of students may make bullying-type comments. Some teachers will have "mailboxes" for students to give each other affirmation cards at anytime. I don't see this as feasible when you have 100+ kids.
I will also have them do these right before we switch seating chart. They have to write an affirmation card to the person on their right and on their left. If they are at the end of the row maybe in front/behind. This assures that everyone receives at least one card. They can also write one to any other person they want in the room that they feel moved to affirm that day. I try to keep them to no more than 5 or you will take all period writing these! If they want to write to more, I give them as many as they want and tell them to write them and bring them back tomorrow and I'll be sure the recipient gets them. For the most part, they are genuine in writing these. For some odd reason I had a student who didn't get one and another students stepped up and said he would write one for her. That prompted other students to write one, too. It really catches on. Kids know what it feels like to get one and they want others to have that feeling as well.
I have also printed off grades and wrote affirmations on every grade report. Yes, some are harder than others, but I really try to find something positive for everyone. They really enjoy getting these.
With the change of seating chart, it's a good time to do it. Some teachers will change their seating chart every 2 weeks! Personally, I don't like rewriting the seating chart this much, but I will change a couple times in an quarter. So, it doesn't have to be often, just enough that they don't get bored with it so that it's still novel, and yet it still means something to them.