> No. To be "in hospital" or to go "to hospital" means that you are a patient. To be "in the hospital" describes only location.
>>> in UK English, not so much American English
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> Your analysis is fine, but does not contradict my comment and moreover is irrelevant to the present discussion, which pertains to the subtle use of articles in English.
As you can see from the dialectal difference you've already described, what you're talking about isn't really related to "the subtle use of articles in English". American English and British English agree on how their articles are used. Where they agree on "in school" and disagree on "in hospital", you can recognize that usage in those phrases, rather than revealing some kind of subtle point about how the is used in nearly-identical contexts, is determined arbitrarily.