Regular verbs follow the rules, and it's a good thing they do. Makes life easier. The world being what it is, however, there always exists a certain amount or order and a certain amount of chaos. This even applies to regular verbs.
It will be important to carefully walk your students through five variants in the conjugation of regular verbs. It all really has to do with the sounds of things; in this case the vowel and consonant combinations in any word that produces a specific sound(s) and spelling. When sounds become unnecessarily difficult, awkward, or even just slow in its execution, people invariably look to break the rules.
In the rules contained within the lesson "Regular Verbs - Rules", you'll find the five variants. The differences between them are to be found in how the past tense deals with final consonants in a verb, the spelling and sylable addition in the third-person present tense, and how the present participle varies with its ending "ing".