Agenda is the appointment diary included with the Series 3 computers. The obvious use is to log meetings etc. but Agenda also lends itself marvelously to timetable and lesson planning!
Set up each lesson as an appointment using Psion-e on the day view, using a unique code. If you run a weekly or two-weekly timetable, press Psion-r to set up a repeating appointment. Make sure you set an end date (eg 31/07/xxxx). Make the repeat weekly, and the period either 1 or 2. Then, to set each lesson, go to the lesson, press enter and type the details. Press enter, and change the second dialogue to "this occurrence only". All the other lessons will be left unchanged.