The headmaster along with his colleagues ____ coming here today.

• With, together with, as well as, along with, in addition to, accompanied by দ্বারা কোন Noun বা Pronoun যুক্ত হলে verb টি প্রথম Subject অনুযায়ী হয়। 
• The headmaster এবং his colleagues, along with দ্বারা যুক্ত হওয়ায় The headmaster অনুযায়ী verb টি singular হবে অর্থাৎ is হবে। 

Source: Advanced Learner's English Grammar & Composition by Chowdhury & Hossain.