Every admission letter is conditional on completing senior year at the level that earned it. Colleges receive a final transcript in June and they do read it. Offers are rescinded every year — not many, but the students it happens to are always surprised.
A single B where there was an A is not the issue. A collapse is.
Act in February, not in May.
Email the admissions office before dropping anything. Colleges admitted you partly on the strength of the schedule you listed. Most will accept a well-explained change; almost none accept finding out in June. This is a two-sentence email and it removes the entire risk.
Contact the college proactively and explain, with any context that matters — illness, a family event. Colleges are far more willing to work with a student who told them than with one they discovered. Proactive contact is the single thing that most improves the outcome.
You are not being asked for a perfect semester. You are being asked not to stop. Keep attending, keep submitting work, keep the grades in the range that got you in.
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