The National Codes Centre at Bletchley Park (who know a thing or two about ciphers) have comprehensive lesson plans, teacher's notes, and student activities for various types of encryption, from simple substitution ciphers (Caesar ciphers) through the infamous Enigma and Lorenz ciphers to modern day methods.
Very detailed but clear explanations of the various types of encryption systems, from simple substitution ciphers (Caesar ciphers) to modern public key encryption systems. The page also explains the types of attacks that can be performed against ciphers, such as known plaintext attacks and frequency analysis attacks. A variety of interactive applets held improve understanding.
This is a kinesthetic lesson designed to help students understand how public key encryption systems work, as well as the types of security challenges such systems address. The plan includes a list of required materials and a video of the lesson being taught (this is a great help in understanding how things are supposed to work). The lesson plan is quite complicated and so is probably better suited to older students, but the analogy (padlocks) is a great way of explaining what can be a fairly confusing concept.
A short video from Computerphile which explains the weaknesses of symmetric key encryption systems and how asymmetric key (public key) encryption solves them. The video explains how public and private keys can be used for secure communication, and how the private key can be used to authenticate the sender (digital signing).
SonicWALL Phishing IQ Test is a quiz game which is a great way to test students' understanding of phishing scams. Players are presented with a series of mock emails, web pages, or URLs that utilize some of the techniques online criminals often use. Spotting the the genuine pages and avoiding the fakes is harder than you might think!