Is School a preposition in this sentence?: The School day never goes too slowly. If it is not what part of speech is it?
It is not a preposition. "School day" is a compound noun. In this case it is the combination of two nouns (school+day), but can also be the combination of multiple different elements (i.e., noun+verb, adverb+noun, etc.). Compound nouns are sometimes written as two separate words (e.g., "bus stop"), as a hyphenated work (e.g., dry-cleaning), or as a single word (e.g., bedroom). In fact, you will sometime see it written as "schoolday."
No, it is not a preposition.
In this use, "school day" is a compound noun.
More specifically than it being a compound noun, or noun phrase, we can say what the role of school is in school day. It is an adjunct noun, or attributive noun. It is describing the sort of day being spoken of.
The question has been answered correctly already. "School" is not a preposition. "The School day" is a compound noun.
Collins Dictionary [www.collinsdictionary.com] defines a preposition as: A preposition is a word such as 'by', 'for', 'into', or 'with' which usually has a noun group as its object."
You can browse a full, comprehensive list of all English prepositions on the web at www.thefreedictionary.com