The national school chaplaincy program will soon come under a new line of attack. Despite the Gillard government's $222 million expansion of the program in the budget, the scheme may yet be struck down as unconstitutional by the High Court, which this month gave the go-ahead for a challenge to be heard in August.

Australia's constitution does not separate church and state. The closest it comes is in section 116, which says the Commonwealth cannot make any law for ''establishing any religion'', ''imposing any religious observance'' or ''prohibiting the free exercise of any religion''.

That section also says ''no religious test shall be required as a qualification for any office or public trust under the Commonwealth''. A Queensland parent, Ron Williams, is relying on this in mounting his challenge. He argues the program is invalid because it sets a religious test for anyone who wishes to be a Commonwealth-funded chaplain.

The government's funding guidelines support his case. A chaplain need not possess expertise or training in counselling, despite providing guidance to students on ''human relationships'' and assisting ''student welfare services''. Instead, a chaplain is appointed due to ''formal ordination'', ''endorsement by a recognised or accepted religious institution'' or the like.

But the High Court's record of cases on section 116 suggests a tough battle. In the 110 years since the constitution came into force, every argument that a federal law breaches the section has failed.