Pupils at seven schools could have shorter days after a trust head say he may have to cut hours to save money.
Dave Baker, of the Olympus Trust which operates near Bristol, said government cuts had left it at "crisis point".
Parents described the funding problem as "diabolical". A union said the idea was "terrible" but families deserved to know the truth about funding.
A Department for Education (DfE) spokesman said new funding plans would increase the trust's budget by 2.4%.
The Olympus Trust runs one secondary, one all-through school and five primary academies in South Gloucestershire, to the north of Bristol.
In the letter Mr Baker warned parents of the "unthinkable" decisions the trust could be forced to make from September.
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