The Tavistock-based school lost 31-15 at home to Surrey’s St George’s Weybridge in the last eight on Wednesday afternoon.
They had overcome St Laurence School from Bath and Torbay Boys’ Grammar School to reach the quarter-finals.
Mount Kelly started well in their big game played in front of a large crowd, but St George’s scored on their first foray into their half on 11 minutes and by half-time had opened up a 17-0 lead.
However, with light fading fast, the home team pulled a try back 15 minutes into the second half through powerful forward Ben Dunlop, who scored from 40-metres out following a scrum.
That try stirred St George’s into life again and they scored two tries within eight minutes to take a 31-5 lead.
But Mount Kelly showed plenty of spirit to finish strongly and score two tries of their own in the last five minutes through Leon Horn and an opportunist 40-metre effort from speedy winger Marco Balzanelli.
“I think this is the furthest we have got in the competition,” he said.
“And for us to be competitive all the way through the game and put 15 points on a school that is three-times the size of us and who play on a circuit of big London schools, is huge bonus for us and a big step forward for our rugby.
“We have a record number of boys playing rugby for school now and Mr Reyburn our new coach this year has been fantastic.
“It was also great to have so many pupils and staff from the school supporting.
“I think this could be a real springboard going forward as we look to become one of the biggest schools in the South West for our rugby.”