While you aren’t required to warm breast milk before using it, some little ones seem to prefer it au natural-like. Doing this while traveling, whether for school kid field trips or upcoming holidays, requires a little thought but is totally doable.
Frozen milk should be thawed in the refrigerator; easiest is overnight. Once you have started feeding your baby a bottle of milk, use and discard within about an hour. You can also travel with a bottle of milk using an insulated bag & ice pack, which can store it safely for 4-5 hours.
These are general tips, of course. For specific situations or questions pertaining to your child, ask your IBCLC.