Equal protection of the laws is a principle found in the Fourteenth Amendment added to the Constitution after the Civil War. Thus the Constitution and laws prohibit actions by a school system or school officials that discriminate and harass against students on the basis of race, religion, or national origin. [1]  Schools cannot exclude children of undocumented or “illegal” immigrants from public schools. [2] In general, school officials shouldn’t even ask a student about their immigration status, since it isn’t relevant to enrolling in school and taking part in academics, athletics and other activity.

[1] Legal separation of races in segregated schools was outlawed a half-century ago in the famous Supreme Court decision, Brown v. Board of Education, 347 U.S. 483 (1954) and 349 U.S. 294 (1955), on grounds that separate education was inherently unequal and violated the guarantee in the Fourteenth Amendment in the Constitution that no state may deprive any persons of “equal protection of the laws.”  Discrimination by any part of government based on race, religion, or national origin was outlawed by Title VI of the Civil Rights Act of 1964, 42 U.S.C. §2000a; Maryland: COMAR § 13A.08.01.16 (requiring involvement of students regardless of sex, race, creed, or national origin); DC: 5 DCMR § 2401.9 (outlawing discrimination in schools of all the kinds included in the D.C. Human Rights Act).
[2] Plyler v. Doe, 457 U.S. 202 (1982).