US Schools and the myriad of decisions made within them are governed (in large part) by data. Everything from test scores, to demographical data, to surveys administered to families in a district is used to determine funding which in turn results in what teachers and students experience on a daily basis.
With this data, states make legislative and policy determinations which in turn affect how districts will manage the particulars of ESL and SEI in any given school. Historically, some of these laws have resulted in increased attention to making equitable access for ELs in public schools. Others have been part of "political tides" restricting instruction to be "English Only" as happened in the early 2000s nationally. What is important to note is what rights ELs and their families for ESL, SEI and other services and the role you will play in ensuring that these rights are upheld.
ESEA "Meaningful/Equitable Access to Education (1964 fact sheet)
ESSA (2015)
Ballot Question #2 -English Only in MA (2002)
RETELL (2011) & Brief (independent) Summary of the Law & its Impact
LOOK Act (2017) & Summary of MTA's Position on the Act
Seal of Biliteracy (2017)
Use this guide from Learning for Justice on how to support immigrants' rights from a teacher/volunteer position.