Probably not too many. Many immigrants are not very well educated in Spanish. They can read and write, but grammar and spelling are pretty weak. Children from Spanish-speaking families can be taught correct Spanish in Spanish class, if they think it’s useful.
One can’t blame the failure of native English speakers to use the language correctly on immigrants. Students are simply not being taught to read English effectively and write it correctly, as a deliberate policy of the “education” establishment.
I wasn't trying to blame as much as point out that perhaps the immigrants may be overwhelming the system so that the school systems can't keep up on the standardized tests. But I don't know the statistics in Texas so I posed the question.
Remember also that Spanish books are written in the urbane syntax of Mexico City and other Latin America metropolis’, not in the country “Mexican” of the mestizos they push our way. In addition we’ve seen tribal Indians come up here who cannot speak fluent Spanish, even in country context.