Tiene is the conjugation of the verb tener for the third person singular in the simple present. The simple present tense is used to talk about habitual actions or routines. This tense is also used to describe things that happen now or in the near future. What this sentence means is that the school has a lot of students. Finally:
La escuela tiene muchos estudiantes
Está is the conjugation of the verb estar for the third person singular in the simple present. We use the verb estar to indicate temporary states and locations. Here this verb is being used as location, describing where my father is temporarily, that is, he's at the school. Finally:
Mi papá está en la escuela
This sentence also stands for the third person singular. Therefore, we also use the conjugation está of the verb estar for this person. Here this verb also stands for location, describing that my son is temporarily at the university. Finally, the complete sentence is:
Mi hijo está en la universidad
Es is the conjugation of the verb ser for the third person singular in the simple present. We use the verb ser to talk about permanent or lasting attributes. In this way, this verb stands for descriptions, occupations, characteristics, time, origin and relationships. Here this verb is being used as description, talking about the essential qualities that describe the nationality of a person. Finally, the complete sentence is:
Mi amiga es colombiana
