I am sure the OP was applying carefully targeted Glyphosate, and using the paint as an aid to be sure of exactly where he was placing the herbicideGlyphosate not legally banned from schools grounds in the UK but councils may ban glyphosate if they wish from any of their premises. Banned from French school areas I believe.
I don't think glyphosate is approved for use in line marking. Have you found a product with an on-label recommendation? The only herbicide I can recall as being approved was glufosinate-ammonium, Kurtail Gold, now withdrawn.