These two experiments are probably most suited to Advanced Higher – possibly for an investigation.
They both involve burning the tea and extracting the iron/manganese in the form of a coloured compound that can be determined colorimetrically.
Iron & manganese in tea -Pupil Guide
Iron & Manganese in Tea – Teacher’s/Technician’s Guide
Analysis – Iron in tea, and tablets – Risk Assessment
Tea Calibration graph
Image by André Karwath used under a Creative Commons License CC BY-SA 2.5