It's not straightforward to do polymorphic joins: one common pattern is to have child tables for each case of the union, but there's no integrity constraint such that each parent must only have one child, e.g.

  CREATE TABLE schools (id SERIAL PRIMARY KEY);
  CREATE TABLE colleges (id INTEGER NOT NULL REFERENCES schools (id));
  CREATE TABLE high_schools (id INTEGER NOT NULL REFERENCES schools (id));

How can you ensure that a school is either a college or high_school but not both?

Another alternative is to make one big table with check constraints but that's also hairy in its own right:

  CREATE TYPE school_type AS ENUM ('college', 'high_school');
  CREATE TABLE schools (
    id SERIAL PRIMARY KEY,
    type school_type,
    /* college columns */,
    /* high school columns */,
    CHECK (type = 'college' AND /* college column constraints */),
    CHECK (type = 'high_school' AND /* high school column constraints */)
  );

The other thing in the grandparent's comment that's a constant pain in SQL is representing an ordered list: how do you insert items into the middle of the list? Depending on your database, it can also be painful to renumber the other items.