I was tempted for a bit to go into a long spiel about how to design worlds and ensure consistency and etc etc etc, but... well, the bit RomanFanboy just said about not sweating things is simply better for your purposes than anything I would have come up with.
note - the genre you are emulating started out a story arc at a time, and only built backstory when it had to. Throw in a few Chekov's guns that feel appropriate to your half-built cosmology (I see that you already have pretty blatant elemental themes for your schools, for example, so maybe tie things into elements and/or colors a bit) and go. Make your characters interesting and fun to write, warp the world to throw them into interesting situations, and don't be afraid to let yourself come up with explanations after the fact (if, indeed, explanations are to be had at all).
In terms of stuff to throw at them, not so much firm guidance as a few ideas...
- Monsters with powers that you want to see them in fights with, and power sources (possibly only vaguely associated) that you think would either be interesting or humorous.
- Theme exploration with completely unjustifiable parallels. If today's theme is snakes, then maybe have one of the hero squad have a fear of snakes and another one has just bought a pet snake that she's quite fond of from the recently opened snake supply center down the road - inspired by a recent visit to the zoo where they all saw the snakes and the phobic friend displayed their phobia. Then have them fight a snake-based monster of some sort, in a battle where the one character's fondness for her pet snake and the other character's phobia of snakes both play significant roles. Stuff like that (though you don't have to go into that much detail). Basically though, once you have a theme for the episode, feel free to work it.
- Lots of stuff about team social dynamics, both in and out of battle, and at least a bit about how being a magical girl/boy affects their home life.
note - the genre you are emulating started out a story arc at a time, and only built backstory when it had to. Throw in a few Chekov's guns that feel appropriate to your half-built cosmology (I see that you already have pretty blatant elemental themes for your schools, for example, so maybe tie things into elements and/or colors a bit) and go. Make your characters interesting and fun to write, warp the world to throw them into interesting situations, and don't be afraid to let yourself come up with explanations after the fact (if, indeed, explanations are to be had at all).
In terms of stuff to throw at them, not so much firm guidance as a few ideas...
- Monsters with powers that you want to see them in fights with, and power sources (possibly only vaguely associated) that you think would either be interesting or humorous.
- Theme exploration with completely unjustifiable parallels. If today's theme is snakes, then maybe have one of the hero squad have a fear of snakes and another one has just bought a pet snake that she's quite fond of from the recently opened snake supply center down the road - inspired by a recent visit to the zoo where they all saw the snakes and the phobic friend displayed their phobia. Then have them fight a snake-based monster of some sort, in a battle where the one character's fondness for her pet snake and the other character's phobia of snakes both play significant roles. Stuff like that (though you don't have to go into that much detail). Basically though, once you have a theme for the episode, feel free to work it.
- Lots of stuff about team social dynamics, both in and out of battle, and at least a bit about how being a magical girl/boy affects their home life.