Jane Reed, head of Down View school, Brighton, has been found guilty of unacceptable professional conduct by England's General Teaching Council.
Although the amount was not revealed in the hearing, the sum involved is believed to be in the hundreds.
She admitted making errors in claiming travel expenses from the local authority as well as outside bodies such as the National Children's Home Adoption Panel and the Learning and Skills Council.
But the GTC found allegations that she had moonlighted while contracted to work for Brighton and Hove city council unproven.
Mrs Reed, a highly regarded headteacher of 26 years' experience of children with special needs, had worked at Down View since 1994.
The GTC heard that Chris Sevink, chair of governors, raised concerns with the city council about Mrs Reed undertaking unauthorised outside duties.
This included work for the LSC, the panel for children's adoption, the Office for Standards in Education and Cambridge Education Associates.
Ian Poole, from the National Association of Head Teachers, represented Mrs Reed at the GTC hearing in Birmingham last week.
He said governors were aware of her extra activities and even encouraged her as they felt her work would promote the reputation of the school.
Mr Poole said the activities did not contravene Mrs Reed's contract with Brighton and Hove city council.
The GTC heard that she claimed travel expenses from the school for attending meetings between April 2001 and December 2002 which were inappropriate, as these were paid to Mrs Reed by the bodies.
She also claimed travel expenses from the school for four meetings of the Standing Advisory Council for Religious Education, which she did not attend.
Mrs Reed admitted making claims for travel expenses in error.
She was suspended in February 2003 as a result of investigations and an auditors' report into her activities between 2001 and 2002.
Bulvinder Michael, chair of the GTC committee, said the panel was satisfied that governors were aware of Mrs Reed's activities and accepted there was no dishonesty in her travel expenses claims.
It found no evidence to support claims that she had failed to observe her suspension by removing papers from the school or going in to use the computers.
But Mrs Michael said: "The travel claims amount to unacceptable professional behaviour. She (Mrs Reed) accepts what she did with the claim form was unacceptable.
"Mrs Reed is an excellent and dedicated head, as the last Ofsted report showed. The school is highly effective and management by the head was excellent. We cannot afford to lose heads of this calibre." She is now teaching at a small, private special school.
The GTC's decision as to her punishment will be announced at a later date.