The advice from the Schools Accountancy Team is that schools should not treat school milk transactions any differently to other catering income/expenditure.
Therefore income from paying parents should come under Schools Catering Income (82001).
Expenditure should come under a code covered by E25 in CFR (for example 34380 Catering Services or 31005 Catering Supplies).
Schools can if they wish set up a separate cost centre on their FMS system to keep track of all income and expenditure relating to milk.