In his biography of O'Connell, Douglas Slawson affirms that in 1928, O'Connell ordered the exhumation of the Sulpicians and transferred the remains to another diocesan seminary.

< declared unfit for priesthood by the Sulpicians because of his inability to master Latin.>
That's probably not the reason O'Connell left the Sulpician seminary.  O'Connell attributes it to health problems brought on by working too hard.  Slawson suggests that O'Connell may have left, or been expelled, subsequent to his having been the victim of what today we might call "bullying."  He belonged to an informal group of seminarians to whom the other seminarians referred as the "Sewing Circle."  
Slawson's book is "Ambition and Arrogance".  It is functionally, if not formally, a sequel to James O'Toole's more complete biography, "Militant and Triumphant."
Persons who want to understand why Boston became the "epicenter" of the abuse crisis must read both of these books.