Six out of 10 primary pupils in the county at the beginning of this year were in classes of at least 30 - and the position is worse now.
Privately, headteachers admit to the authority's education chair, Dave Wilcox, that they have classes of 40-plus, but none will go public.
For in the "money follows pupils" climate created by local management of schools, they fear that parents will withdraw their children and leave behind an even deeper financial crisis. This has already happened to those schools that were brave enough to speak out.
Six years ago, Derbyshire had one of the best pupil:teacher ratios in the country. Now it languishes at the bottom of the table.
Among the poorest-funded education authorities in England, it has the highest number of primary children in classes of at least 30. The past year has seen an increase of around 10 per cent in the percentage of primary pupils in classes of 30 or more, and almost a fifth are now in classes of 35 or more.
Mr Wilcox blames Government-imposed spending cuts and a higher-than-average number of children with statements of special educational need. In the past two years at least 500 teachers have been made redundant because schools could not afford to keep them on, forcing the authority to make huge payouts. The proportion of Derbyshire children who have statements is 3.7 per cent while the national figure is slightly above 3 per cent.
Schools in the county this year gained a 2.1 per cent budget rise, after a two-year cash freeze. It is unclear what will happen next year, but the county is already looking at cuts of between Pounds 6.8m and Pounds 13.8m.
"If we could see some improvement in class size I think it would just give such a boost to everyone," said Mr Wilcox. "They would at least then think there was some light at the end of the tunnel.
"We have schools with classes of 40. They confess them to me, but won't do so publicly. We have had a few schools that have gone public but they have lost students and money as a consequence."
Paul Tozer, chair of the Derbyshire Association of School Governors, added: "When the Government says things like the Budget was a budget for education, it really sticks in your throat. Classes should have fewer than 30 children. If small classes are good enough for Eton, why can't they be good enough for our children?"