covariant.scala

scala> class Person
defined class Person

scala> class Bob extends Person
defined class Bob

scala> class Alice extends Person
defined class Alice

scala> class School[+A]
defined class School

scala> def in(s: School[Person]) = s
in: (s: School[Person])School[Person]

scala> in(new School[Person])
res0: School[Person] = School@16b4a017

scala> in(new School[Bob])
res1: School[Person] = School@6f79caec

scala> in(new School[Alice])
res2: School[Person] = School@6fffcba5

scala> class Animal
defined class Animal

scala> class Cat extends Animal
defined class Cat

scala> in(new School[Cat])
<console>:13: error: type mismatch;
 found   : School[Cat]
 required: School[Person]
              in(new School[Cat])
                 ^

scala> in(new School[Cat with Bob])
res4: School[Person] = School@4f970963