At my breaking point

I’m engaged to my FH who has 3 boys and 1 girl from previous relationships. Currently I’m struggling with his 2 teens 12 and 15. The 15 year old is in many school activities and stressed or so he says. Recently we started a chore chart as I will be going from WFH back to in person as I won’t be able to do all the housework as often anymore. Recently the 15 yo boy just comes home from school activity’s grabs food and goes to his room. He doesn’t come out until I’m in bed. I feel like I’m doing something wrong. It hurts so bad because he was the one that welcomed me with open arms when his dad and I 1st started dating. Last night we talked to him about grades bc he had missed assignments so he has an F. He’s like it’s not really a F. I said if it says it’s on paper it’s an F. I’m so frustrated from the lack of respect I just feel like odd person out and always feel like the bad guy.