It’s halfway through the school day for Sofia Lucero, Lilhyann Ortiz and Anai Orona, and that means it’s time for snacks and recess. The 4-year-olds gobble down colorful fruit snacks at a picnic table before racing up, down and around the playground. They chatter in English and Spanish among themselves and with three teachers supervising the fun. When recess is over, they line up single file and hop back into their classroom — a 20-foot bus. The girls attend a preschool on wheels, a new program from Thornton-based educational organization Right On Learning.