The gift by Simon, 57, of Indianapolis, is the largest in the school's history and the second-largest ever made in the United States to an independent school, Mercerburg Academy said.
Simon, a 1974 graduate, previously gave the school $5 million for a new student center and $3 million for a theater. Her family is prominent in the retail shopping center business and founded Minnesota's Mall of America, the Pennsylvania news website pennlive.com reported.
The gift announced Thursday will be mainly dedicated to scholarships and to support faculty research and development, Academy spokeswoman Katie Derickson said.
The school enrolls boarding and day students in grades nine through 12.
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