There was no evidence offered about the first language of the hispanic students. It wouldn't be a matter of "losing their Spanish". It would be a matter of not getting any further instruction in a language they understand. The article only cites improvements in scores relatives to their peers. How do they perform compared to students at that grade level across the country based on standardized exams?
I always try to read the black words on the white page:
“I think the parents think … our students going on to middle school are going to miss out on their Spanish, lose much of what they’ve gained. This program needs to keep expanding.”
Now, exactly what part of that quote is hard to understand?
The parents are not afraid that the students will not understand math, they are afraid they will lose the mastery of spanish they have gained.
Whether you choose to believe it or not, mastery of spanish is a valuable skill. There is significant trade between english-speaking nations and spanish-speaking ones. A mastery of both languages is of great practical significance for anyone involved in any aspect of this trade, and it becomes even more valuable as one ascends to higher levels of the organizations carrying out that trade.
Do you really want to cede all of the high positions in a number of businesses to people born speaking spanish and schooled speaking english only, or would you like to have some native english-speakers schooled in dual-language programs in contention?