Secondary roads are numbered by the number of the county followed by a unique number for the particular road. Interstates Primary Auxiliary (three-digit) Business Routes US highways State routes Secondary roads Rail CSX Transportation and Norfolk Southern are the only Class I railroad companies in South Carolina, as other freight companies in the state are short lines. Amtrak operates four passenger routes in South Carolina: the Crescent, the Palmetto, the Silver Meteor, and the Silver Star. The Crescent route serves the Upstate cities, the Silver Star serves the Midlands cities, and the Palmetto and Silver Meteor routes serve the lowcountry cities. Major and regional airports There are seven significant airports in South Carolina, all of which act as regional airport hubs. The busiest by passenger volume is Charleston International Airport. Just across the border in North Carolina is Charlotte/Douglas International Airport, the 30th busiest airport in the world, in terms of passengers. Columbia Metropolitan Airport – Columbia Charleston International Airport – North Charleston Greenville-Spartanburg International Airport – Greenville/Spartanburg Florence Regional Airport – Florence Myrtle Beach International Airport – Myrtle Beach Hilton Head Airport – Hilton Head Island/Beaufort Rock Hill/York County Airport – Rock Hill Education As of 2010, South Carolina is one of three states that have not agreed to use competitive international math and language standards. In 2014, the South Carolina Supreme Court ruled the state had failed to provide a "minimally adequate" education to children in all parts of the state as required by the state's constitution. South Carolina has 1,144 K–12 schools in 85 school districts with an enrollment of 712,244 as of fall 2009. As of the 2008–2009 school year, South Carolina spent $9,450 per student which places it 31st in the country for per student spending. In 2015, the national average SAT score was 1490 and the South Carolina average was 1442, 48 points lower than the national average. South Carolina is the only state which owns and operates a statewide school bus system. As of December 2016, the state maintains a 5,582-bus fleet with the average vehicle in service being fifteen years old (the national average is six) having logged 236,000 miles. Half of the state's school buses are more than 15 years old and some are reportedly up to 30 years old. In 2017 in the budget proposal, Superintendent of Education Molly Spearman requested the state lease to purchase 1,000 buses to replace