Teenagers given a Q in their GCSE results had to wait seven days to find out their fate.
An unknown issue that caused the delay meant Lord Grey students got their English exam results a week after the rest of the country.
This meant that Chloe Wilson, 16, and her fellow pupils at the Bletchley school missed the deadline to confirm their place at sixth form.
Mum Emma said: “The school’s communication has been shocking. They weren’t able to tell us what had gone wrong, or when we would be likely to get the results back.
“The only reason I found out that Chloe could pick up her envelope for the second time was because I saw a notice on the school’s website.”
The exam board, Cambridge International, failed to disclose the issue that caused results to be delayed for seven days.
A spokesman said: “The results of students at Lord Grey school were delayed due to an issue that Cambridge International Examinations was investigating with the school.
“This issue has since been resolved and all students have received their results.”
Lord Grey students returned to school on Thursday to pick up their results - but didn’t find out what the letter Q stood for.
Ms Wilson added: “It must stand for query, but no one ever told us.
“My daughter was at risk of losing her place at her chosen sixth form - yet we haven’t even had an explanation as to what went wrong.