To me, the use of plural (have) suggests that you have a personal involvement in the group: you are one of the students of which every second has been expelled. The singular 3rd person inflection is free of such connotations. If you ask me, in case of group noun-verb agreement, the verb form to be chosen only comes down to whether there is some personal involvement or not.
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AnonymousHi ! The answer to the question lies in the sentence itself. One should always look at the object of preposition. for ex in this sentence fifty percent of the students; students is plural, so it will be followed by plural verb,which is have in this case. Further, if the objct of preposition is singular then it'll be followed by singular form of verb.