Well you should have told that you are student.
. Avoid Madam Tussads then, it is expensive but you may get some offer (like Tussads + London Eeye for 24 quid, but I still think they are overrated, depends on you really.)
Some more museums:
1. Science museum
2. Victoria and Albert museum
(Both these and Natural history museum are in one area)
3. National art gallery (guess its free not sure)
London is pretty safe, but be careful at night, teen gangs are scary.
There are two more places you can consider visiting outside london (stonehenge and bath)
Some useful websites.
gumtree.com
tfl.gov.uk
nationalexpress.com
nationalrail.co.uk
megabus.com
Get an oyester card as soon as you touch down. There could be student concession for many things, also for travelling together. Ask before you buy a ticket.