Check the small print!
The umbrella will charge a "processing fee" per payslip, sometimes quite hefty. Many will take employers NIC off your pay - as said above, someone has to pay the employers NIC, the school won't because you're not their employee, the umbrella won't pay it out of their fee, so they usually deduct it from your payslip, i.e. you pay.
The dispensation they claim allows them to pay your expenses is a benefit to them, not you. It's only purpose is to allow them to pay expenses without them seeing proof of payment or checking allowability. If HMRC challenge, it comes down to you to satisfy HMRC that your claims are genuinely allowable and that you can prove you spent the money you've claimed. The umbrella won't help you - it's not their problem. Luckily dispensations are all withdrawn from 6/4/16, so the umbrella's can't market this anymore.
The hourly rate is higher because no-one has the associated costs of employing you, such as holiday pay, sick pay, maternity pay, redundancy, pensions, or other benefits. You could find yourself very disadvantaged if something nasty happens to you - you basically have to throw yourself at the mercy of the state, rather than being looked after by an employer.
If you research properly and understand all the pros and cons, then you can make an informed decision. Trouble is, a lot of people just see the headline figures and fall for the smoke and mirrors. There are countless threads on internet forums from people just started with an agency/umbrella and shocked at seeing deductions for umbrella processing fees and employers NIC from their wages! Be aware.