Five fifth-graders from Milton, Vt., are heading to Washington to help Michelle Obama plant the White House garden this spring. School district officials said last week the invitation was prompted by the district’s changes to its food service. Obama had noted a blog post from Milton food service director Steve Marinelli about the fresh entrees now available to Milton students, along with fruits and vegetables in their new ‘‘self-serve’’ bars. Obama also liked the students’ essays about how changes in the school food service helped the school and community. WCAX-TV reported that the students were to leave on Wednesday and will be at the White House on Thursday.