Here is what the ACLU Handbook _The Rights of Students_ 3rd Edition,
1988, says about access by both students and "outsiders":

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[Question:] Can students use school facilities such a bulletin
boards, loudspeakers, mimeograph machines, and meeting rooms to
express their views?

[Answer:] If the use of these school facilities for expression of
student views would not be likely to disrupt regular school
activities, students should be able to use them. However, a school may
legitimately refuse to allow students to use its mimeograph machine
if, for example, it is used all day long in the preparation of course
material. If the loudspeaker is used only for school business, such as
the announcement of school activities, program changes, and special
events, a court would probably hold that student groups may legally be
prevented from using for the expression of particular opinions. On the
other hand, facilities such as bulletin boards should present no
problem since space for the use of students can almost always be made
available.

Of course, if any facility is made available to one group, the school
may not then deny other groups the opportunity to use that facility.
Thus, a court held that it was unconstitutional for school officials
to refuse to distribute notices informing patents of probusing rallies
when those officials hand previously agreed to distribute notices
about antibusing rallies.[13]

In another case, an antiwar organization had been denied permission by
the school board to provide students with information regarding legal
alternatives to the draft and military server even though
representatives of the armed services were permitted to provide
students with information about military careers. In some schools, the
military recruiters were permitted to distribute literature, post
notices, place ads in school papers, conduct workshops, provide
counseling, and administer tests. The court held that the school
board's policy was unconstitutional because it favored one point of
view over another. The court ordered that the antiwar organization be
given the same access to students and school facilities as was given
to the military recuiters.[14]

In 1984, Congress passed the Equal Access Act, which makes it unlawful
for any public secondary school to deny to any student group wishing
to conduct a meeting equal access to school facilities on the basis of
the content of the speech at the meeting.[15] Although there have been
few cases interpreting that act, it may be that the act will
strengthen the right of students to use school facilities for
meetings.[16]

[References]

[13] _Bonner-Lyons v. School Committee of Boston_, 480 F.2d 442 (1st
Cir. 1973). See also _National Socialist White People's Party v.
Ringers_, 473 F.2d 1010 (4th Cir. 1973); _Zucker v. Panitz_, 299 F.
Supp. 102 (S.D.N.Y. 1969); _Garvin v. Rosenau_, 455 F.2d 233 (6th Cir.
1972), _on remand_, Civil Action No. 36093 (E.D. Mich. 1972); _A.C.L.U.
v. Radford College_, 315 F. Supp. 893 (W.D. Va. 1970).

[14] _Clergy and Laity Concerned v. Chicago Board of Education_, 586
F.  Supp. 1408 (N.D. Ill. 1984).

[15] 20 U.S.C. SS4070 _et seq_.

[16] _Student Coalition for Peace v. Lower Merion School District_,
776 F. 2d 431 (3d Cir. 1985)