Low time private pilot reported realizing he was flying too low over a populated area.

Narrative

I am a private pilot and I was doing a local solo flight to fulfill a requirement for solo time for my commercial certificate. I fly with a flight school that is part of a larger university. This morning I was getting the aircraft preflighted to go on my solo and I had the idea to do multiple flyovers of my main university campus. After doing my runup I advised our tower frequency what I would be doing and they approved and I also got clearance into the St. Louis class B that lays above it. The Campus area I was flying over was in the middle of a city in a widely populated area so the MSA would be in the higher 1000s MSL and close to 2000. The shelf that lays over the campus was at 2000 MSL however I still wanted to get clearance. TRACON approved my flyovers at 2000 MSL; however the lowest I got was around 1250 feet and the highest was 1900. When I was at this low altitude; I violated the minimum safe altitude for a populated area on multiple occasions when I descended below it. The reason I wanted to ask for B clearance in the first place was that I would have been able to go above that 2000 if needed. When I thought about it I had the altitude 1800 MSL in mind as I would be clear of the 1000 ft above the highest obstacle clearance. After takeoff when I started to fly west I was at 1200 but then as I got closer; I climbed to 1900. I was focused on what was going on outside and I believe this was the primary cause of losing altitude to go below the MSAs and not paying as much attention as I should have to the altimeter. Another contributing factor I believe was not paying enough attention to the actual elevation the ground was above sea level. The field elevation I was departing was at 400 MSL; however the area I was overflying was closer to around 600. Needless to say I understand I had the potential of putting others and myself in danger during this flight with the altitudes I was at. If somebody were to do a similar activity; then they should get clearance into the Class B and not go below the 2000 if no other traffic is conflicting and keep a close eye on altitude. After noticing how low I was; I climbed up to an altitude I thought would be acceptable. Most of the remainder of the flight I was cruising around our local practice area to process what had happened.