Your school system, under IDEA and its state counterparts, is required to fully evaluate any child who may need special education services ""in all areas related to the suspected disability, including, if appropriate, health, vision, hearing, social and emotional status, general intelligence, academic performance, communicative status, and motor abilities."" (34 CFR Sec. 300.532)
Before the school does so, and before providing or changing special education services, it must notify you in writing. For the first evaluation and placement, schools must also obtain parental consent. IDEA's requirements for parental consent vary depending on whether the LEA is seeking an initial evaluation or a reevaluation and on whether the parents affirmatively respond to a request for consent, simply do not respond, or cannot be located.
IDEA (20 USC Ch 33 Sec 1414 (c)(3)) provides that an LEA must ""obtain informed parental consent . . . prior to conducting any re-evaluation of a child with a disability, except that such informed parent consent need not be obtained if the local educational agency can demonstrate that it had taken reasonable measures to obtain such consent and the child's parent has failed to respond.""
Thus, while an LEA may proceed to re-evaluate without parental consent, that is true only if it has first taken reasonable, documentable measures to obtain consent. This means the LEA must be able to show documents such as records of attempts to call the parents, correspondence to and from the parents, and/or records of visits to the parents' home or place(s) of employment. (Per 34 C.F.R. sec. 300.345(d)) If parents do respond, but affirmatively refuse to consent to the LEA's re-evaluation, the LEA would have to seek an order to override the parents' refusal to consent. (34 C.F.R. sec. 300.505(b))
For an initial evaluation, it appears that even with documentable reasonable efforts to obtain consent, if the parents do not respond, the LEA cannot go ahead with the evaluation without further steps. In that case, if the reason consent could not be obtained is that the parents cannot be identified or located, presumably the LEA could seek the appointment of an educational surrogate (see 20 U.S.C. sec. 1415(b)(2)), or seek an order from the due process agency (presumably, this would be a ""matter relating to the identification, evaluation, or educational placement of the child"" and thus within the agency's jurisdiction.) If parents respond but refuse to consent to the initial evaluation, the LEA can seek an order from the due process agency to permit the evaluation. (34 C.F.R. sec. 300.505(b))Your never-ending role
As a parent, you must make sure that all areas of possible need are assessed as quickly as possible. While some parents would rather not allow their school system to evaluate their child, a refusal to cooperate at this stage of the process can backfire if you need to ask for more or for different services later. It may also affect your ability to have the school system pay for an independent evaluation.