these parts:
Amazing what lengths people will drive themselves to for some food, no let me correct that, its amazing how STUPID my sister is for only putting effort into such things.
“Its amazing how stupid you are to put such effort into getting into this school just for the food”
“How can you call me stupid right to my face all of a sudden!?” Mayumi yelled with tears in the corner of her eyes
the first two parts are redundant or rather, confusing. The way it is narrated, the dialogue of what he thinks of his sister is unnecessary. In a way, it is better that the 'thought' was immediately followed by the sister's dialogue to better express that the mc did not intentionally say it out loud.
it would have been good if you described how (directions) they went to school, designating important landmarks/signposts to give a general feel of the neighborhood, since its the beginning, the author needs to give a panoramic description of every new location outside and inside. I would like to have known where and how the MC parted with his sister before reaching the classroom. Also, mind that it will help the reader if you described the interior of the school, specifically the main hall and corridors as well as the location of the mc's classroom and important stairwells.
need a physical description for Tabuki, whether or not he's an important character is irrelevant. This goes for every other character. On Chihiro, its straight bangs trimmed to just above her eyes. otherwise people would assume the bangs are swept to one side. again hair color would be nice. any additional detail like accessories and mannerisms is more than welcome.
again, do describe the classroom, seat arrangements, classroom equipment, number of tables, MC's and other characters' seat locations, wall detail, etc. are important. After reading, I'm lead to believe that the three of them are still talking at the door of the classroom.