Former Highlands and Islands MSP Dave Petrie, 64, suffered a heart attack before collapsing in front of his shocked Trinity Academy colleagues on August 25.
He was immediately rushed to Edinburgh Royal Infirmary, where he died six days later on Thursday.
His brother-in-law Duncan Munro paid tribute and told how his death had come “completely out of the blue”.
He said: “He was a big strong man, fighting fit, with no real health problems.
“Dave was looking forward to heading off to New Zealand for the Rugby World Cup with my two sons, Ian and Neil. They are both even considering not going themselves now. The entire family are devastated.”
Before entering politics, Mr Petrie had worked as a manager at Scottish Water and only retrained as a teacher in 2003.
He had been working at Trinity Academy as a supply maths teacher for the past eight months, a school which he himself attended as a child.
Mr Petrie stayed in Trinity with his sister, Anne McKenzie, during the week, returning to his Oban home and wife of 40 years, Grace, each weekend.
Trinity Academy headteacher Alec Morris told of his shock at his death.
He said: “The entire school is devastated. He was very popular amongst both staff members and pupils. One moment he was speaking with a colleague, preparing for the day ahead, and the next he had keeled over and collapsed.
“He was far from a doddering old teacher, Dave was buoyant and energetic with a real love of rugby and was excited about his upcoming trip to New Zealand.
“He had that unique ability to get on with people immediately as soon as he met them. He will be sorely missed. It’s so sad.”
Mr Petrie became an MSP after Mary Scanlon stood down from Holyrood in 2006 to contest the Moray by-election triggered by the death of Margaret Ewing. He followed Scanlon on the Conservative Party list for the Highlands and Islands after previously standing as a candidate for Argyll and Bute.
However, he did not win re-election at the following 2007 Scottish Parliament election.
Scottish Conservative leader Annabel Goldie today said: “This is very sad news. Those who knew him well have lost a much loved family member and friend.
“The Conservatives mourn the passing of a true friend and ally.
“When we needed him, he stepped in and made a valuable contribution to our parliament.”
A keen tennis, squash and rugby fan, he is survived by his daughter Gill, 26.
As a mark of respect, Mr Petrie’s local rugby club Oban Lorne, of which he was a former president and captain, cancelled their fixtures last weekend.
His funeral will take place at 1.30pm in St Ronan’s Catholic Church, Corran Esplanade, in Oban on Wednesday, September 7.