In what would be a shakeup of Austin’s public company hierarchy, student housing company American Campus Communities Inc. said April 19 it has agreed to be purchased by funds controlled by Blackstone Inc., a giant in alternative investment management. The all-cash deal would pay $65.47 per share and is valued at about $12.8 billion, including ACC’s debt. That price represents a 22% premium compared with the 90-calendar-day average of the stock price and a 30% premium compared with its closing on Feb. 16, before American Campus indicated it was open to a deal.