Not a teacher.
If you can’t remember the difference, use a mnemonic hook:
Principal is always Number One, i.e. the most important person in an organisation. Think of a prince.
Senior just means older in a sense of long service/more experience, as opposed to someone junior, i.e. someone who has only been in a job or position for a short time.
There can be many Senior Lecturers, but there is only one Principal Lecturer.