One of the first prayers your students learned in their Catholic education was probably the prayer to their Guardian Angel:

Angel of God

My guardian dear

To Whom His love
Commits me here
Ever this day
Be at my side
To light and guard
To rule and guide. Amen

As the conclusion of the semester nears, pray these words with your class and remind your students to call on their Guardian Angels for assistance in their studying, taking of exams, and with any challenges of their lives. 

Also:

• Review Church teaching on angels from the Catechism of the Catholic Church. 
• Review for yourself a simple lesson on Guardian Angels from Jared Dees.
• Play for the students a short two-minute video “5 Amazing Things about Our Guardian Angels.”
• Have your students compose their own prayers to their Guardian Angels.
• Display images of Guardian Angels (as in this post). Have your students draw a contemporary  image of their Guardian Angel protecting them in some daily aspect of their life.