Complex sentence: From 1967 until 1994, the abbey ran a prep school for boys aged 7 to 13, but was obliged to close it as the school became financially unviable due to dwindling numbers of boarders.
Split into simpler sentences: From 1967 until 1994, the abbey ran a prep school for boys aged 7 to 13. The Abbey was obliged to close it as the school became financially unviable due to dwindling numbers of boarders.