Je vais à l'école à pied.
Let's break it down:
* Je = I
* vais = go (present tense of "aller")
* à = to
* l'école = school (definite article "le" changes to "l'" before a vowel sound)
* à pied = on foot
You could also say:
* J'y vais à pied. (literally "I go there on foot"). This is a more informal way of saying it, and uses the pronoun "y" to refer to "school."