Yup. It's very important to do this. You'll get clumps of secondaries, so like... 3 schools will send you secondaries one day, then you'll get 2 more the next day, then 3 more a couple days after that. It's difficult to keep up with them all without it getting backed up. If the essays are prewritten, you'll be able to knock out the secondaries as they come.
Also (fortunately), many of the secondary essays for the schools are very similar, so some only require small tweaks. Yes, the same essay can be used for different schools (just make sure you use the appropriate school name if you mention it in your essay).