Well actually, FIRST you should simmer the milk, not bring it to a boil but just so you see small bubbles on the rim and fumes on top. This will kill the unfriendly bacteria (you would not want THEM to reproduce).
Only add the probiotics once the milk has returned to room temperature (you could test it on your wrist as for a baby bottle).
Plus if you keep everything warm (ex put the mixture in a jar, covered with a cloth of put into an insulated lunch box), the lactic bacteria will reproduce faster.