“ The imposition of a heavier tax burden on lessees of federal property than is imposed on lessees of other exempt public property must be justified by significant differences between the two classes. The School District addresses this problem, essentially, as one of equal protection, and argues that we must uphold the classification, though apparently discriminatory, 'if any state of facts reasonably can be conceived that would sustain it.' Allied Stores of Ohio, Inc., v. ”

“ However, all lessees of exempt public lands would appear to belong to the class defined by Article 7173. [6] In view of the fact that lessees in this class are taxed because they use exempt property for a nonexempt purpose, they appear to be similarly situated and presumably should be taxed alike. Yet by the amendment of Article 5248, the Texas Legislature segregated federal lessees and imposed on them a heavier tax burden than is imposed on the other members of the class by Article 7173. ”

“ In those cases we did not decide-in fact, we were not asked to decide-whether the exemption of school-owned property rendered the statute discriminatory. Neither the Government nor its lessees, to whom the statute was applicable, claimed discrimination of this character. [11] Since the issue was not raised, the basis for the separate classification of property owned by schools was not examined. Therefore, the Michigan cases shed no light on the classification problem here. ”

“ City of Detroit, supra, 355 U.S. at pages 472, note 2, 474, note 6, 78 S.Ct. at pages 477, 478.↑ Only issues raised by the jurisdictional statement or petition for certiorari, as the case may be, are considered by the Court. Supreme Court Rules, 15, par. 1 (c) (1) , 23, par. ”