I checked the chapter on Alvah in " Schools and Schoolmasters of Banffshire". Unfortunately for your purposes (prior to 1806), the first headmaster named is a Mr. Charles Begrie, who was headmaster for some years at the start of the 19th century. The author states that education in Alvah is not mentioned in the Statistical Account. Also in the section on Alvah is a brief section on Dunlugas and Linhead, both of which have mention of some lady teachers, but not of the name you are looking for. In the section on Forglen, a schoolmaster named Mr. M'Killigan in 1785 is mentioned. No similar names in Marnoch. Have you come across any other part of Banffshire in any of your documents?
Names in Alvah include: Ruddiman, Chapman, Andrew Todd, Allan Rennie Andrew, Andrew , Mr. Alex Stuart Wilson, Rev James Duncan, Mr. Robert Hepburn, George Pirie, Mr. John Kidd
I will have a look in the Gamrie chapter and see if I can come up with anything about your ancestor.