に and で: what happens at a place
に and で after a place are not interchangeable variants. Use the verb to distinguish where something exists from where an activity takes place.
に for where something exists
図書館に学生がいます。 means 'There is a student in the library.' With いる, the library is a place of existence. 本が机の上にあります。 similarly locates a book on a desk.
In 駅に着きます。 ('I will arrive at the station'), に marks the destination reached. Learn this separately from location of existence, even if your language translates both with the same preposition or particle.
で for where an activity happens
図書館で勉強します。 means 'I study at the library.' で marks where studying happens. The same library takes a different particle with います and 勉強します because its role in the sentence changes.
公園で祭りがあります。 means 'There will be a festival in the park.' Although the verb is あります, this describes an event taking place, so で is used. A location before ある does not always require に.
Keep other uses separate
In 七時に出ます。, に marks the departure time, seven o'clock. In バスで行きます。, で marks transport by bus. Read the relationship between the preceding phrase and the verb rather than assigning each particle just one meaning.
Translate 'be at school' and 'study at school,' then substitute another location. Finally, try an event such as a festival to check that you are not choosing the particle solely from the verb's written form.
Check: Which describes a festival taking place in a park: 公園に祭りがあります or 公園で祭りがあります?
Show the answer and why
公園で祭りがあります。 is natural for the location of an event. Distinguish this from に locating a person or object.