By Raquel Castillo and Sonya Dowsett MADRID (Reuters) - Kelvin Heredia dropped out of school aged 16 during the heady days of Spain's real estate boom to take up well-paid work as a window fitter. Now, a decade later, Heredia is unemployed and without qualifications, a plight that highlights two intractable problems holding back Spain's economy - the highest youth unemployment rate in the European Union, a ranking it reclaimed from Greece in June, and the highest school drop-out rate. Youth unemployment remains a big headache for Prime Minister Mariano Rajoy's centre-right government as it prepares for what promises to be a closely-fought national election on Dec. 20.