Thirteen may be unlucky for some, but recently this number showed just how far the rugby programme within the school has come in the last few years.
That is because over the weekend of March 15, 16 and 17 there were 13 rugby players with connections to the school representing Scotland.
Last Friday, Matt Davidson was at full-back when Scotland played England at under-20 level in Northampton to finish off their age-grade Six Nations campaign.
Winger, Rufus McLean, was also part of the under-20s squad for that tournament and would have been involved, but for injury.
The two former pupils have now returned to their respective clubs London Scottish and Watsonians as they begin working towards selection for the age-grade World Championships this summer.
The following day three other former pupils were with Scotland at Twickenham.
Magnus Bradbury, recently back from a long-term shoulder issue, started at No.8 and played a big part as Scotland bounced back from 31-7 down to end up with a 38-38 draw at the home of English rugby.
Bradbury scored a cracking try, while hooker, Fraser Brown, came on in the second half and added some much needed energy to the visitors.
Stafford McDowall, last year’s Scotland under-20 captain, was also at Twickenham as 24th man to give him a taste of the big time.
And given the season the centre is having with Glasgow Warriors it can only be a matter of time until the 21-year-old earns a full cap.
On Sunday, eight current school pupils were in action for national age-grade teams.
Two Scottish under-16 squads – the Reds and the Blues – played Newcastle Falcons’ youngsters in a triangular event in the north of England.
And Merchiston’s Teddy T, James L, Ben A, Rudi B, Guy N and Adam W were all involved.
In April, Scotland under-16s will head down to Wales for a tournament and the boys are all in the mix for the tour.
Also in April, Scotland under-18s will be in Gloucestershire to play in the Six Nations Festival.
Jamie D, Matthew C and Dan G played for the under-18s in their warm-up game against England in Billingham on March 17, and all Merchiston boys scored tries during the game.
Players with Merchiston Castle connections involved with Scotland between March 15 and March 17
Magnus Bradbury, Fraser Brown, Stafford McDowall
Jamie D, Matthew C, Dan G
Teddy T, James L, Ben A, Rudi B, Guy N, Adam W