CHARLTON – The state is ordering oil giant ExxonMobil Corp. to test 42 private home wells near three public schools to assess a migrating plume of groundwater contamination caused by a large gasoline tank explosion more than 25 years ago. In addition, the Telegram & Gazette has learned that two schools, the Charlton Middle and Heritage schools, have had small detections of a possible carcinogenic gasoline chemical in their pre-filtered drinking water for years.