Hi. At your age
calcium tablets will not be beneficial. Usually in this age group, back strain and pain are due to issues with posture.
Next Steps
Please try to do back flexibility exercises and lumbar spine stretching and strengthening exercises. These are easily available online. You can contact me if you have difficulty.
Health Tips
1. Use a lumbar pillow (small pillow while sitting in chair)
2. Hold mobile devices at eye level
3. You can use Tablet MOBIZOX One tablet twice a day for 3-4 days and stop
4. Tablet Pregalin-XSR 75mg one tablet after dinner for 2 weeks. It may cause some drowsiness so to be taken at night