Re-enrollment into public schools:
Texas public schools are required by law to accept all school aged children residing in Texas, including homeschooled kids, regardless of their home education. The TEA has this to say on the subject.
Elementary:
Schools typically use age as the primary factor in placement. They might ask for samples of the student's work or test them. For example, if a kid is 9, they could be in third or fourth grade. After testing, the school can decide which is a better fit. The testing and samples can also be used to determine if the student might benefit from special services.
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Secondary:
Secondary schools depend heavily on a credit system. For this reason, if you are enrolling a kid for high school or during high school, the school will very likely want them to prove competency for any credits by passing an End of Course exam, showing college credit, acquiring credit from a recognized institution, etc. Because the law allows schools flexibility, it is best to contact the district first.
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These are good resources for preparing for such examinations.