Who are ‘porridgemates’?
As you might guess, ‘однокашники’ (‘odnokashniki’ or ‘porridgemates’) are people who ate the same porridge. Ozhegov's dictionary says that this word is actually used to mean "comrades in training and education".
It’s believed that the word appeared in the 19th century. Perhaps, its roots are in the tradition of rural schools to serve milk porridge at the end of the school year.
There is, however, another version. Porridge was one of the main dishes in the diet of regular people, especially students at gymnasiums, colleges, universities and other educational institutions. So, people united by a common cause or study, with whom they shared a meal, began to be called ‘porridgemates’.
An English equivalent would be: ‘Messmates’.