To add to David's list, there are missing births from Brian's index (especially for Peel) at:
lawsonsiom.freevar.com/ under "Brian Lawson's registered births index".

Also, depending whether he was born before or after April 3rd/4th 1881, the transcript of the 1881 census, taken at that date, is freely available on FamilySearch.org.
Even if he wasn't yet born you should have his parents' names from Brian's index, so you could find them on the census with his older siblings.

There was a William SANDERSON birth on 8 Nov 1879 in Peel with parents William SANDERSON & Ann Jane CORLET (and Robert b.25 June 1878 Peel, same parents).
Are you certain of his birth year?

For Peel schools see Manx Notebook (link above) and put Peel schools at the Search facility.

Sue