Searching through the net, I came across this latin-mass-society.org/calnan.htm 
BEFORE MASS
Arrive in the Sacristy ten minutes before time.
If only all our servers would do this, you will often see them doing a run and genuflect, because they are late (and sorry ladies, but is always the girls 
)
Funny, it is always the boys in my parish and as the sacristan I see it all the time.
Why would a sacristan have a better view of this than a typical “pew-sitter”?
Because I am the one who most often lets the alter servers in a sees what time they really arrive and I am the one who lights the candles when the alter servers coming running into the sacrasity with scant minutes to spare.