This is the 812th most frequent Czech word.
bývalá
"Ex" or "former" (feminine form).
The word 'bývalá' describes something that existed in the past but does not exist in the same form anymore. Here, it refers to a previously existing or former school that has been replaced.
Bývalá škola byla zbořena a nahradil ji nový moderní komplex.
The former school was demolished and replaced by a new modern complex.
In this sentence, 'bývalá' means a person who was in a former relationship with the speaker. Usage depends on the context; here, it denotes an ex-girlfriend.
Potkal jsem svou bývalou přítelkyni v kavárně.
I met my ex-girlfriend at the café.
Here, 'bývalá' is used to describe the house as having had a previous role or function (a post office) and now serving a different purpose.