A basic outline, icon or drawing of an opened door. 

Opened Door Outline

A science class blackboard with drawings such as an uterus, a science flask, a virus and a heart shape.

Blackboard With Science And Biology Drawings

A blackboard with many education elements drawn on it, like letters, jagged lines, molecules and books.

A handwritten symbol of a square root next to the number three as blackboard content for math class.

An icon, drawing or outline of a formula, molecule or chemical bonds. 

An outlined drawing or icon of an ink pen inside a rectangular frame. 

A math class blackboard with equations and scribbles.

A blackboard with many history elements drawn on it, such as roman dates, planet Earth and a Chinese temple. 

A square 4x4 checkered pattern or chess board.

A minimalistic geometric, square-ish human body drawing, outline or icon.

A minimalistic icon, drawing or outline of closed books in front of an open door as chalkboard content for litterature class.

An art class blackboard with drawn elements such as faces, a music note, fish and a musical instrument.

A simple dove drawing or icon, such as chalkboard content for art class. 

A busy and overworked manager or employee surrounded by charts, graphs, and notifications.

A blackboard complete with geometry explanation and drawings, such as a right triangle, a pie chart and an equation.