This is impressive. Here is the only, or at least one of the only, edutainment browser games I've ever seen. It gives me renewed interest in the possibility of browser games, which to be honest rarely set a lasting good impression on me in the early 2000's.
Each other item I can think of to say in praise has already been said by users who have already posted here. The gameplay itself, the way you interwove the lessons with the gameplay. I will also add that the general presentation is well-executed. These little letters, numbers, and symbols feel alive as they fight, right down to the AI. It even feels tragic watching the other passwords fall, and the survivors back away into corners only to be mowed down.
If I may leave a little constructive criticism, a couple of things.
First of all, kids should be introduced to the logic and importance of password protection early in life, and this game would be perfect- and yet I personally think it's important let kids keep clean language as long as possible. Please consider replacing the word "pissed" with "ticked" and otherwise letting profanity be given a miss.
Secondly, the accomplishment of defeating all the other passwords is fairly significant in my opinion. I would like to advise adding something after defeating the last password. Even just a text block saying , in more words, "Congratulations! Remember that these principles apply in real life, so feel free to use what you've learned here in any passwords you make going forward!" This would bookend what has otherwise been a unique and enjoyable edutainment experience.
Third, the ranged attacks of the numbers are easily dodged just by the back-and-forth the other team's symbols do by default. This makes numbers feel like they're missing out on their potential. The best I can suggest is a very slight tracking system, or just having the arrows come out horizontally.
Fourth, the simplicity of combat makes the point-and-click movement feel unrealized in its potential. If you ever want to integrate additional concepts into password security education, please consider working them into the arena so we have something more to click on than the other guy's password symbol troupe.
This is all in love. You've made a darn good browser game here, and edutainment game. Please keep up the good work!