What does the expression "the school first XI rugger team" mean? [closed]

I found this sentence in a novel set in Cornwall. The complete sentence is:

Daddy is very proud that you have made the school first XI rugger team.

The boy in the team il 16 y.o. and he lives in the Thirties of the Twentieth century.

Can you help me understand the meaning of this expression?

Rugger often stands for Rugby Football, usually called Rugby, in informal English. And that makes sense in combination with team.
'Made the team' means 'joined the team'.

The first team is the best of the teams in the school.

And XI stands for the number of players in the field, 11 in this case, so a reduced size team, as explained in a comment by @KillingTime. Or it may be that the boy made the XV team and somewhere it got an error like the mother understood it wrongly and used the wrong number, and the V became a I. The actual team will be bigger but this is the number playing at one time.

Even when allowing for the time of writing, this sentence is surly non-standard, it sounds like 'poetic' rather than 'everyday' use of English.

No sources for this, I have learned all these words from my English friends many of whom have played rugby.