Notice, it may seem like he's taken quite the leap in logic to automatically assume she doesn't care about him at all, but let's notice some details here:
-He's both sleep deprived and overworked, so he probably isn't thinking completely rationally
-He's pretty clearly depressed, which also clouds judgement and leads to obsessive guilt usually
-He's already stressed from being called out by the teacher, having to do work throughout the night, and feeling betrayed, meaning he's more likely to burst
Other notes:
-Notice how his mother tries to wonder where the money he spent has gone, and she almost implies that she knew it wasn't going to the food stand. She's gathering information
-We see more indications that Sensei is realizing MC is being abused throughout the piece
-The twisted thing about Yuri this chapter is that this act isn't a front and probably is her genuine feelings. She probably did want to comfort MC on some level despite feeling happy about his changed view on Haru. What's insidious is that it's her that planted the seed of doubt in the first place, and then she finds the fact he's grown into her biases as vindication of her beliefs. She doesn't offer to help mend the friendship or to fix him, but she's acting as if he's come to the light and has redeemed himself. It's almost like SHE forgives HIM for straying away from HER ideal path for him, and so goes on to try and kinda convert him into what she wants him to be.
@animepig wait until you read
Kimi ni aisarete itakata. It's a well-crafted story but damn is it fucked up. I still get chills thinking about one scene in particular. (You'll know what I'm talking about.)