Hi! You will need to break the ball's velocity down into its vertical and horizontal components.
The vertical component is equal to 15m/s times sin(50˚). The horizontal component is 15m/s times cos(50˚).
Given the horizontal component of the velocity and the distance of the kicker from the posts, you can work out how long it will take the ball to reach the posts, since the horizontal component won't change with gravity (and we're ignoring air resistance presumably).
Then you can plug the amount of time you've just derived, the initial vertical component of the ball's speed, and acceleration due to gravity (9.8 m/s/s towards the ground or whatever approximation your class is using) into the right SUVAT equation to determine how high the ball will be when it reaches the posts. Remember that if we're defining positive vertical distance as up, acceleration due to gravity should be considered negative.
Good luck!