Tom Noon told the annual conference of the National Association of Schoolmasters Union of Women Teachers in Peebles that his authority declined to pay his full salary after he was badly injured on a sanctioned trip to the United States.
Mr Noon suffered a ruptured knee tendon during a knock-about football match at Nasa space centre in Houston during a trip with 25 Scottish pupils in September 2004. He was off work for eight months and went through two operations to rebuild his knee.
Glasgow City Council paid his full salary for six months but in line with local conditions of service reduced it to half pay for a further two months before Mr Noon was able to return to work at All Saints Secondary.
"I'm pretty annoyed about it, so I'm reluctant to become involved in extra-curricular activities again. This could happen to any teacher involved in any aspect of extra-curricular activities. It's the lack of local authority insurance cover in the event of an accident that's the problem," Mr Noon said.
He has 30 years' experience of activities behind him and as a mountain leader has regularly taken pupils away on trips. "It's a personal thing,"
Outlining his difficulties, he cautioned: "Teachers might be covered for hospital costs, transport and ambulances, and even loss of limbs, but in the event of a non-disabling injury longer than six months you may not be covered for loss of earnings when you go down to half pay."
Mr Noon argued that it should not be left to individual teachers to take out extra insurance. At present, it was "at the discretion" of the authority whether to pay out.
The union has now agreed to write to all authorities to find out precisely what cover they have.
A spokeswoman for Glasgow City Council said: "Our position as regards this particular case was reached after extensive discussions with, among others, the council's corporate human resources and legal sections. We accepted the advice given to us and acted accordingly."