This paper presents and discusses a discourse relation annotation scheme for the MUCH corpus of academic writing, based on Rhetorical Structure Theory (RST). The set of proposed relational tags takes into regard both distinctiveness, pedagogical needs and implementability with automatic rules. We show how a pilot grammar with 180 rules can map discourse relations between existing syntactic nodes, exploiting lower-level grammatical/treebank markup and surface clues such as connectives (e.g. conjunctions and prepositions). In an evaluation of a live run on student essays from teacher training courses, the average false positive rate across the most frequent 21 categories was 26.7 % for tags and 17.1 % for relation links. Performance was best for categories with a high percentage of rules using surface connectives and, for in-sentence relations, their corresponding dependency links.