Teach the circle of fifths. Start with the key of C, which has no sharps or flats. By going up five notes you get to G, whose scale has one sharp. Going up five notes from the starting note brings a new scale and one sharp more for each new note. When you have taught the sharp scales to B, it's time to introduce the concept of enharmonics, two notes that sound the same but are spelled differently. So the key of B with five sharps sounds the same as Cb with seven flats. Progressing around the circle of fifths, each note that is five steps higher than the one before has one less flat. Other enharmonic scales are F#/Gb and C#/Db.
Explain that all major scales are made by connecting two tetrachords with a whole step. A tetrachord is a series of four notes made with a whole step between the first and second notes and the second and third notes, followed by a half step between the third and fourth notes.
Teach students to memorize scales one scale at a time. Start with the C major scale with no sharps or flats. Once that scale has been committed to memory, add the F scale with one flat and don't progress any further until that scale has been learned. Follow the F scale with the G scale with one sharp. Proceed adding sharp and flat scales one at a time until the students have mastered scales with four sharps and four flats. After the students have become proficient on these scales, the remainder of the scales can be taught.
Present minor scales to students after they're able to play all the major scales that have up to four sharps and four flats. There are three main forms of minor scales: harmonic, melodic and natural, but the harmonic is the form that most instrumentalists will need to learn first. The harmonic minor has the third and sixth steps lowered by a half step, and an interval of a step and a half between the sixth and seventh steps. The example of a C harmonic minor scale would be C, D, Eb, F, G, Ab, B and C. Even though in C minor the key signature would be three flats, only the E and A would be flatted and the B would be B natural.
Teach the more advanced students the pentatonic scale with five notes and the six note whole tone scale. A pentatonic scale composed of a whole step followed by another whole step. The space between the third and fourth note is a step and a half followed by a whole step. The whole tone scale is just a scale with a whole tone between each note. There's really only one whole tone scale, but it can start on any note.