<div class="FormField__input">
  <div class="Checkboxes">
    <% @contexts.each do |c| %>
    <div class="Checkbox">
      <input type="checkbox" class="Checkbox__input" name="lesson[contexts][]" value="<%= c %>" id="lesson_contexts_<%= c.id %>" />
      <label class="Checkbox__label" for="lesson_contexts_<%= c.id %>">
        <%= c.name.titleize %>
      </label>
    </div>
    <% end %>
  </div>
</div>

<div class="FormField__input">
  <div class="Checkboxes">
    <% @contexts.each do |c| %>
    <div class="Checkbox">
      <input type="checkbox" class="Checkbox__input" name="lesson[contexts][]" value="<%= c.id %>" id="lesson_contexts_<%= c.id %>" />
      <label class="Checkbox__label" for="lesson_contexts_<%= c.id %>">
        <%= c.name.titleize %>
      </label>
    </div>
    <% end %>
  </div>
</div>

Use id of context as value
Use id of context as value
