I am an 8th grader applying to prep school for next year.
Lets say my ECs, essays, and recommendations are all good. I got a 91th percentile on my SSAT. But in the beginning of last year, I got bad grades (online school) and I was wondering how much this would affect my chances if at all.
How are you defining “Bad Grades?” And how were the final grades?
First quarter, I got high/low Bs and two Cs 
But final grades I got As/Bs and this year I have all As. I also mentioned how I struggled with online school during my interview
Your improvement will be noted. Good luck.
Having a bad stretch of grades right before applying would definitely be a negative in your application if there weren’t extenuating circumstances. But this particular scenario has been extremely common and admissions officers will be familiar with it. In fact, many schools (very likely including some you are applying to) recognized that converting to distance learning during the covid shut-down was so difficult that those semesters were made pass/fail or left out of GPA calculations. They understand that those grades are not representative of you as a student in an in-person class.
Somewhere in your application (there may be a good built-in spot but you can also often attach an additional statement) you’ll want to explain that those semesters were converted to online during the pandemic and you found the format difficult. You can note when school went back to in-person and that grades returned to (your) normal then. You can also use this to show some self-reflection… What did that experience with online school teach or clarify for you about what type of academic environment you’re looking for? Small classes? Peer discussion? Interactive lessons? Forming relationships with teachers who really get to know each student? Feeling like you’re part of a community?
Ok thank you so much for explaining!