You should call the DMV mandatory unit in Sacramento @ (916) 657-6525. Be sure that you call them from a Non-California area code. Tell they you have moved out of California, and do not intend to return (if that is the case), and ask them to send you a form expempting you from having to do the alcohol school. The California DMV does not recognize any out-of-state alcohol schools, so you need to get the waiver. If they approve the waiver, they will put a 2-year "hold" on your California license,
This is a question that should be posted under California because California law will apply. You should contact the lawyer who helped you get the probation. They may be able to help. You should also ask your probation officer from California. (In order to be able to transfer to Texas under the Compact Agreement, you must have first gotten permission from California, after they had consulted with Texas.)
To my knowledge, Texas does not have an 18 month "class". The classes related to DWI (DUI) in Texas are significantly shorter.
A good place to start in order to find out how Texas will treat the situation (after you get the California issue figured out) is at www.txdps.state.tx.us or try calling the Drivers License Information center at: Driver License Customer Service: (512) 424-2600.
The personnel at the Texas DPS are always helpful and willing to spend time with you to get your questions answered as long as you do not ask for legal advise.
Look up a Texas attorney in the city you are moving to on ncdd.com for more information when you get to Texas.
A roundup of the best tips and legal advice.