Since it's for school and you have a limited time, I suggest you start by determining the lenght of the game. For a 5 months works and a first project... I suggest you go for a 1 hours Game with some extra explorations maps. To begin, maybe you should write a story... if you don't want to go on complicate, just create a town, a village or a place where action is taking place. You don't really need a ''plot'' to make a game. Think of Tetris... Once you know where the game takes place, you can start writing down who's gonna be the character, try using realy life people, it's funnier and you may even put your teacher in the game. Go simple with 4 or 5 Main Character... so it doesn't get too complex with Dialog, characters movement and Interaction. I strongly suggest you buy yourself a physical NoteBook with paper... it works really well.
Once the characters are done, let the action begin : Dialog, fighting, puzzle, teleportation, exploration, buying stuff, trading stuff... feel free to make anything happens, since it's your first game, let it be as original as it can be. Idea you can use : Story takes place in your own bedroom. You're sitting in front of paper, computer.. and you're explaining with message box that you need to write and create your own game for school.... Alright, you need a paper, so get out of the house and make the player buy a paper... and a pencil... Go back to home... problem, no more light! Make the heroe goes in the basement of your house to get some oil for the lamp... or just some fire... In the basement you meet an old fantome... or a zombie... or just a talking rat... You talk, you talk, blablablaaa... He tell you that he'll give you what you're looking for but he's hungry.... Now you must get some food for that ''guy'' downstairs... and on and on, if you need more inspiration, just ask, maybe I can help you too with the events. I know there's tons of tutorials out there but sometime you don't want to read THE WHOLE book just to realise that the little girl had brown hair... As you saw it, the exemple I just threw was really Linear like. And my advice is that you keep it the most linear possible so you don't get lost in event, fork condition and stuffs that would ruin your first RPG maker experience. If you success at ending your first RPG project, than I garantee you that all your future project will have better chance to see the light. So in big : -Short story -Linear process -Simple Quest or Action -Limited characters and few NPC -Take lots of note on the sheet -DoN't over Brainstorm or it'll end in a storm. And last advice, if you need help, Direct PM to Philoup! Cya! Good luck!