Playing the game the way it was designed, this could take you an absolute age. There used to be a quick method for increaing town value involving ghost hunting, but that has been patched out. Luckily, there is a new one involving pre-teens, and with the devs announcing that there will be no more updates for this game, it should be safe that this one is here to stay. You should probably go for this last, as you will earn a fair chunk from completing goals and playing through. What you can exploit is the fact that every time a pre-teen gets to A+++ grade, you get a town value increase of 30k. Build yourself a room with as many 3* study desks as you have preteens. They cost 3LP each, but if you can't spare the LP, then 2* desks will do, but it will be a little slower. Upgrade them when you can.
So, first get your pre-teen(s) to A+++ grade, then set them to do a single brainstorm session, lasting 3 minutes. When they have started studying, make them leave school and re-enrol. You should get the bonus for getting to A+++ grade at the end of the three minute session, meaning 30k per 3 minutes per student. The more students you have, the more money you can make in that 3 minutes. The important thing is to stay with the students. Do not pause the game, leave the room they are in, and to be on the safe side, just leave the phone. Make sure your timeout settings for your phone are at least 3 minutes, as that will pause your game.
Sometimes it won't work properly, or you will get interupted in some way. This will bump your student down to A+ or A++, but shouldn't take long to get back up to where you need them. Way easier and quicker that doing as they intended!
Thanks to Fire Hawk D for informing me of this method.