My advice to you if you are moving here with school-aged children:
Don't wait until you get here before you start immersing them in a good language program. Do that now. Why? If/when you move here you are going to want to put your kids in school at some point, even if you homeschool the first year after you arrive. Our kids are now enrolled in a private school and are doing well socially. But one of our sons is going through a bigger adjustment academically than we thought would happen.
Por ejemplo (for example): He is doing the same level of math this year in his Spanish curriculum that he did in last years English home-school curriculum with a few things added in. But he is adjusting to some aspects of the this years course study more slowly because they are speaking on a more advanced level with regards to Spanish verbs in the command form. Now his teachers are telling me that his Spanish is improving. But when we are studying and doing his homework in History, Geography, Ethics and Civic Studies....whatever, it is taking longer for us to decipher the written text in those classes because much of the vocabulary and verb conjugation is new to both he and I. Mexican History, as an example is obviously going to be spoken almost exclusively in the past tense. That means the verbs fluctuate between preterite and co-pretertie with regards to all all five verb tenses of me, you, him/her, them and us.
And both he and I have finished the first three and one-half levels of Spansh studies from http://www.learnables.com/. Even if your kids have taken some of these courses, which I strongly advise that you do in advance of moving here, there is still an adjustment period for them in the language and in their course study. Some nights I spend two hours translating text with my son just to get him to a point where he can read and decipher the content of his course studies. In doing so we utilize Google Translate quite extensively along with a good Spanish dictionary.
And on top of that, if your child's teachers are not understanding of the adjustments they are going through and they grade them like they should be fluent, well, let's just say not every teacher in the world understands the word "curve". Some here do and some don't. I have already had at least three sit downs with my son's teachers. We love them and are grateful for the opportunities God has given our children and they are learning and growing. Not everything comes easy with that and you just have to understand that this will involve you with their homework, preparation for upcoming tests and general levels of encouragement more than maybe if they were going to schools where everyone speaks your native tongue.
P.S. Just because a school in Merida says they are bilingual doesn't mean they are really bilingual. The bulk of the course studies will be in Spanish more than likely. If the teacher teaching those courses speaks very little English and they are not understanding of your situation, you will be wishing your kids spoke more Spanish. Study Spanish now.