one-third of Texas school districts did not even meet the state-mandated standards for maximum class size. The great majority of these are low-wealth districts. In many instances, wealthy and poor districts are found contiguous to one another within the same county. Based on these facts, the trial court concluded that the school financing system violates the Texas Constitution's equal rights guarantee of article I, section 3, the due course of law guarantee of article I, section 19, and the "efficiency" mandate of article VII, section 1. The court of appeals reversed. We reverse the judgment of the court of appeals and, with modification, affirm the judgment of the trial court. Article VII, section 1 of the Texas Constitution provides: A general diffusion of knowledge being essential to the preservation of the liberties and rights of the people, it shall be the duty of the Legislature of the State to establish and make suitable provision for the support and maintenance of an efficient system of public free schools. The court of appeals declined to address petitioners' challenge under this provision and concluded instead that its interpretation was a "political question." Said the court: That provision does, of course, require that the school system be "efficient," but the provision provides no guidance as to how this or any other court may arrive at a determination of what is efficient or inefficient. Given the enormous complexity of a school system educating three million children, this Court concludes that which is, or is not, "efficient" is essentially a political question not suitable for judicial review. 761 S.W.2d at 867. We disagree. This is not an area in which the Constitution vests exclusive discretion in the legislature; rather the language of article VII, section 1 imposes on the legislature an affirmative duty to establish and provide for the public free schools. This duty is not committed unconditionally to the legislature's discretion, but instead is accompanied by standards. By express constitutional mandate, the legislature must make "suitable" provision for an "efficient" system for the "essential" purpose of a "general diffusion of knowledge." While these are admittedly not precise terms, they do provide a standard by which this court must, when called upon to do so, measure the constitutionality of the legislature's actions. See Williams v. Taylor, 83 Tex. 667, 19 S.W. 156 (1892). We do not undertake this responsibility lightly and we begin with a presumption of constitutionality. See Texas Public Bldg. Authority v. Mattox, 686 S.W.2d 924, 927 (Tex.1985). Nevertheless, what this court said in only its second term, when first summoned to strike down an act of the Republic of Texas Congress, is still true: [W]e have not been unmindful of the magnitude of the principles involved, and the respect due to the popular branch of the government.... Fortunately, however, for the people, the function of the judiciary in deciding constitutional questions is not one which it is at liberty to decline.... [We] cannot, as the legislature may, avoid a measure because it approaches the confines of the constitution; [we] cannot pass it by because it is doubtful; with whatever doubt, with whatever difficulties a case may be attended, [we] must decide it,