Russell finished the Common School System, then current in the US. That meant he 'graduated' when he finished seventh grade. There after he took course work through a YMCA system.
From Separate Identity, volume 1:
The YMCA gave him the opportunity to
“do some good.” The YMCA was not the social club it is today. It existed to
rescue sinners and to promote Christian work. He joined the Association in 1865
or 1866. The Pittsburgh Association was originally founded in 1854 but had
become moribund. It was reorganized in 1865 and became a social force in the
two cities. The Association offered evening classes in Commercial Law, Public
Speaking and Parliamentary Law, Engineering Mathematics, Arithmetic, Working
Mathematics, Electricity, Metallurgy, Chemistry, Architectural Drawing,
Mechanical Drawing, Freehand Drawing and Designing, French, Spanish, German,
Italian, English, and Spelling, Vocal Music, Bookkeeping, Stenography and
Penmanship. To us this strongly suggests what Russell meant when he said he was
educated by “private tutors.”[1]
The Pittsburgh Association also maintained an active street evangelism. Leland
Dewitt Baldwin in his excellent history of Pittsburgh wrote that the YMCA “fought nobly the vanities of the
world, particularly the theaters.”[2]
[1] Sarah H. Killikelly: The History of Pittsburgh: Its Rise and Progress, B. C. Gordon Montgomery Company, Pittsburgh, 1906, pages 410-411. The claim that he was instructed by “private tutors” was made by Russell in the biographical material provided to The National Cyclopedia of American Biography. See the main entry in Volume 12, page 317.
[2] L. D. Baldwin: Pittsburgh: The Story of a City: 1750-1865, University of Pittsburgh Press, 1937, 1995 edition, page 260.