Concept:
Prepositions are used to show the relationship between words in a sentence. Some prepositions occur in fixed expressions that must be memorized.
Step 1: Identify the expression used to indicate travelling by walking.
The sentence describes the manner in which a person goes to school.
Since the person is walking, English uses the fixed expression:
\[
\text{on foot}
\]
Step 2: Examine the given options.
The expressions
\[
\text{by foot}, \quad \text{over foot}, \quad \text{along foot}
\]
are not standard English expressions.
Only “on foot” is grammatically correct.
Step 3: Complete the sentence using the correct preposition.
The sentence becomes:
\[
\text{He goes to school on foot.}
\]
Therefore,
\[
\boxed{\text{on}}
\]