The article is omitted in prepositional phrases that refer to traveling to places where a change in social behaviors is required.
Hence the pattern "Mary had a little lamb. ... It followed her to school one day" (rather than "to the school") is standard, as is "I'll see you in court" (rather than "in the court"). Most English speakers say "in town" but "in the city". All English speakers say "go to college"; British speakers will also say "go to university" and "go to hospital" (for American speakers, it is "go to the hospital"). These phrases are a matter of custom rather than following clear rule.
In fact, there is continuing debate over the use and semantics of noun phrases with articles. It is more customary to consider the article as 'not used' rather than 'omitted' in these cases, as claiming that something is 'omitted' is to make wider claims about the grammatical system that are far from easy to substantiate. The reason for not using an article is not so much that a change of behaviour is required, as claimed above, but more that the noun phrase under the scope of the article is referred to as an institution as opposed to a particular place. "I'll see you in court" for a court case as opposed to "I'll see you in the court" because this is where we are meeting next. Also, I study "at university" (institution), but left my jacket "in the university" (location). Exceptions, as usual, seem to be the rule, as e.g. "I went to the police station" is used in both senses.